•?*•■'** ■ ’ < M"*'! 

■ • -■-• iiH 1 .Ijt.* «tt t t t*< < I • -t ( I l.t 

• ♦ '*< « •«*.*, VI,' 

'tr* “ m* ♦ -i < v 


M I 

i-mT, I,, 

t*Ar.4 A i.io « * .* 


('<4 j ( li'fM ,t a'i'.r JV‘ 
• Hi»,H 




» «,<•< f4i| m < <•( I'i 


,' ■''{'<•4:* •* »>r* l»4it ■ 

*jt't ♦><'».» .4 4*t 




04 t rt/4'» 

• j4. • r 4 •' -t ^4 • 4 4 


!* i* 4411*9 


. \ ** 7^ T’ 7» *' *1 ’•I n 41 1 *< 

^•A I ♦« 

‘ ' ■ • *»9**'l 



T^BCLASSIFIBD 
By authority SecroUry of 

SEP 14 1960 

Defense memo 2 August 1960 
ubeary’oFcongrbss 




SEP 1 4 19$o 
IHrfeMenmiao2AnOT8tl960 
“®*ABy'oPOTNca»ESS 


DOCUMENT, ALL CLASSTF^f/Tmxr ’ 

markings Mijbt be CANCfet.tisnr 





t 


1 


SUMMARY TECHNICAL REPORT 
OE THE 

NATIONAL DEFENSE RESEARCH COMMITTEE 





1 his document contains information affecting the national defense of the 
United States within the meaning of the Espionage Act, 50 U. S. C., 31 and 32, 
as amended. Its transmission or the revelation of its contents in any manner to 
an unauthorized person is prohibited by law. 


Tliis volume is classified in accordance with security regula- 

tions of the War and Na^^Departments because certain chapters contain mate- 
rial which was at the date of printing. Other chapters may 

have had a lower classification or none. The reader is advised to consult the 
\Var and Navy agencies listed on the reverse of this page for the current 
classification of any material. 



TO 


Manuscript and illustrations for this volume were prepared for 
publication by the Summary Reports Group of the Columbia Uni- 
versity Division of War Research under contract OEMsr-1131 
with the Office of Scientific Research and DcAelopment. This 
volume was printed and bound by the Columbia University Press. 

Distribution of the Summary Technical Report of NDRC has been 
made by the War and Navy Departments. Inquiries concerning the 
availability and distribution of the Summary Technical Report 
volumes and microfilmed and other reference material should be 
addressed to the War Department Library, Room lA-522, The 
Pentagon, Washington 25, D. C., or to the Office of Naval 
Research, Navy Department, Attention: Reports and Documents 
Section, Washington 25, D. C. 



Copy No. 

239 




RESTRICTEDX 


SUMMARY TECHNICAL REPORT OF DIVISION 7, NDRC 


VOLUME 2 


RANGE FINDERS AND 

TRACKING PEGLASSTPron 

^ anUnritgr Secratoiy of 

SEP 14 1980 

fiefmse memo 2 August i 960 

congress 


OFFICE OF SCIENTIFIC RESEARCH AND DEVELOPMENT 
VANNEVAR BUSH, DIRECTOR 


NATIONAL DEFENSE RESEARCH COMMITTEE 
JAMES B. CONANT, CHAIRMAN 


DIVISION 7 
HAROLD L. HAZEN, CHIEF 



WASHINGTON, D. C., 1947 


CLA SSIFIC'A Tin \ t (-t: . 

■authWeti- 




NATIONAL DEFENSE RESEARCH COMMITTEE 


James B. Conaiit, Chdinnan 
Richard C. Tolman, Vice Chairnuni 


Roger Adams 
Frank B. Jewett 
Karl T. Compton 

Irvin Stewart, 


Army Representative^ 
Navy Representative- 
Commissioner of Patents'^ 
Executive Secretary 


'^Army representatives in order of sewice: 
Maj. Gen. G. V. Strong Col. L. A. Denson 

Maj. Gen. R. C. Moore Col. P. R. Faymonville 

Maj. Gen. C. C. Williams Brig. Gen. E. A. Regnier 

Brig. Gen. W. A. Wood., Jr. Col. M. M. Irvine 

Col. E. A. Rontheau 


^Navy representatives in order of sewice: 

Rear Adm. H. G. Bowen Rear Adm. J. Finer 

Capt. Lybrand P. Smith Rear Adm. A. H. X'^an Keuren 

Commodore H. A. Schade 
^Commissioners of Patents in order of service: 
Conway P. Coe Casper W. Coins 


NOTES ON THE ORGANIZATION OF NDRC 


The duties of the National Defense Research Committee 
were (1) to recommend to the Director of OSRD suitable 
projects and research programs on the instrumentalities of 
warfare, together with contract facilities for carrying out 
these projects and programs, and (2) to administer tech- 
nical and scientific work of the contracts. More specifically, 
NDRC functioned by initiating research projects on re- 
quests from the Army or the Navy, or on requests from an 
allied government transmitted through the Liaison Office 
of OSRD, or on its own considered initiative as a result of 
the experience of its members. Proposals prepared by the 
Division, Panel, or Committee for research contracts for 
performance of the work involved in such projects were 
first reviewed by NDRC, and if approved, recommended to 
the Director of OSRD. Upon approval of a proposal by the 
Director, a contract permitting maximum flexibility of 
scientific effort was arranged. The business aspects of the 
contract, including such matters as materials, clearances, 
vouchers, patents, priorities, legal matters, and administra- 
tion of patent matters were handled by the Executive Sec- 
retary of OSRD. 

Originally NDRC administered its work through five 
divisions, each headed by one of the NDRC members. 
These were: 

Division A — Armor and Ordnance 
Division B — Bombs, Fuels, Gases, &: Chemical Problems 
Division C — Communication and Transportation 
Division D — Detection, Controls, and Instruments 
Division E — Patents and Inventions 


In a reorganization in the fall of 1942, twenty-three ad- 
ministrative divisions, panels, or committees were created, 
each with a chief selected on the basis of his outstanding 
work in the particular field. The NDRC members then be- 
came a reviewing and advisory group to the Director of 
OSRD. The final organization was as follows: 


Division 1 — Ballistic Research 

Division 2 — Effects of Impact and Explosion 

Division 3 — Rocket Ordnance 

Division 4 — Ordnance Accessories 

Division 5 — New Missiles 

Division 6 — Sub-Surface AVarfare 

Division 7 — Fire Control 

Division 8 — Explosives 

Division 9 — Chemistry 

Division 10 ■— Absorbents and Aerosols 

Division 1 1 — Chemical Engineering 

Division 12 — Transportation 

Division 1 3 — Electrical Communication 

Division 14 — Radar 

Division 15 — Radio Coordination 

Division 16 — Optics and Camouflage 

Division 17 — Physics 

Division 18 — War Metallurgy 

Division 19 — Miscellaneous 

Applied Mathematics Panel 

Applied Psychology Panel 

Committee on Propagation 

Tropical Deterioration Administrative Committee 


iv 


Library of C'ongress 



2015 


460886 


NDRC FOREWORD 


A s EVENTS of the years preceding 1940 revealed more 
x\ and more clearly the seriousness of the world 
situation, many scientists in this country came to 
realize the need for organizing scientific research for 
service in a national emergency. Recommendations 
which they made to the White House were given 
careful and sympathetic attention, and as a result the 
National Defense Research Committee [NDRC] 
was formed by Executive Order of the President in 
the summer of 1940. The members of NDRC, ap- 
pointed by the President, were instructed to supple- 
ment the work of the Army and Navy in the de- 
velopment of the instrumentalities of war. A year 
later, upon the establishment of the Office of 
Scientific Research and Development [OSRD], 
NDRC became one of its units. 

The Summary Technical Report of NDRC is a 
conscientious effort on the part of NDRC to sum- 
marize and evaluate its work and to present it in a 
useful and permanent form. It comprises some 
seventy volumes broken into groups corresponding 
to the NDRC Divisions, Panels, and Committees. 

The Summary Technical Report of each Division, 
Panel, or Committee is an integral survey of the 
work of that group. The first volume of each group’s 
report contains a summary of the report, stating the 
problems presented and the philosophy of attacking 
them, and summarizing the results of the research, 
development, and training activities undertaken. 
Some volumes may be “state of the art” treatises cov- 
ering subjects to which various research groups have 
contributed information. Others may contain de- 
scriptions of devices developed in the laboratories. 
A master index of all these divisional panel, and 
committee reports which together constitute the 
Summary Technical Report of NDRC is contained 
in a separate volume, which also includes the index 
of a microfilm record of pertinent technical labo- 
ratory reports and reference material. 

Some of the NDRC-sponsored researches which 
had been declassified by the end of 1945 were of 
sufficient popular interest that it was found desirable 
to report them in the form of monographs, such as 
the series on radar by Division 14 and the mono- 
graph on sampling inspection by the Applied 
Mathematics Panel. Since the material treated in 
them is not duplicated in the Summary Technical 
Report of NDRC, the monographs are an important 
part of the story of these aspects of NDRC research. 


In contrast to the information on radar, which is 
of widespread interest and much of which is released 
to the public, the research on subsurface warfare is 
largely classified and is of general interest to a more 
restricted group. As a consequence, the report of 
Division 6 is found almost entirely in its Summary 
Technical Report, which runs to over twenty 
volumes. The extent of the work of a Division cannot 
therefore be judged solely by the number of volumes 
devoted to it in the Summary Technical Report of 
NDRC: account must be taken of the monographs 
and available reports published elsewhere. 

The Fire Control Division, initially Section D2 
under the leadership of Warren Weaver and later 
Division 7 under Harold L. Hazen, made significant 
contributions to an already highly developed art. It 
marked the entrance of the civilian scientist into 
what had hitherto been regarded as a military 
specialty. 

It was one of the tasks of the Division to explore 
and solve the intricate problems of control of fire 
against the modern military aircraft. Gunnery 
against high speed aircraft involves fire control in 
three dimensions. The need for lightning action and 
superlatively accurate results makes mere human 
skills hopelessly inadequate. The Division’s answer 
was the development of the electronic M-9 director 
which, controlling the fire of the Army’s heavy AA 
guns, proved its worth in the defense of the Anzio 
Beachhead and in the protection of London and 
Antwerp against the Nazi V-weapons. In addition to 
producing mechanisms such as the M-9, the Division 
made less tangible but equally significant contribu- 
tions through the application of research methods 
which had a profound, even revolutionary, influence 
on fire control theory and practice. 

The results of the work of Division 7, formerly 
Section D2, are told in its Summary Technical Re- 
port, which has been prepared at the direction of the 
Division Chief and has been authorized by him for 
publication. It is a record of creativeness and devo- 
tion on the part of men to whom their country will 
always be grateful. 

J. B. CoNANT, Chairman 
National Defense Research Committee 

Vannevar Bush, Director 
Office of Scientific Research and Development 


CT ’ - 



"0 


V 



FOREWORD 


I MMEDIATELY UPON its formation, the Fire Control 
Division (initially Section D-2, later Division 7) of 
NDRC focused its attention upon antiaircraft artil- 
lery as the most difficult and tactically the most im- 
portant problem in fire control. Investigation soon 
confirmed the optical range finder as the greatest 
individual instrumental contributor to inaccuracy of 
AA fire. Intensive work was initiated by the Division 
to strengthen this weakest link, and it grew into a 
large, diverse and comprehensive series of investiga- 
tions. Physiological and psychological characteristics 
of range finder operators rapidly emerged as factors 
of the greatest importance and indicated that atten- 
tion must be paid to the man-instrument combina- 
tion. At this point the author of this volume. Dr. 
Samuel Weiller Fernberger, professor of psychol- 
ogy at the University of Pennsylvania with a notable 
record in the field of psychophysical measurement 
methods, was called to the Division to guide the 
psychophysical aspects of this work as the associate 
of Dr. T. C. Fry, the member of the Division who 
carried responsibility for the entire optical range 
finder program of the Division. 

A great volume and range of work was done in this 
field under many auspices including the Fire Control 
Division. A substantial fraction of this work has con- 
tinuing importance as is evident to even the casual 
peruser of this volume. It was not until near the end 


of the war that radar data-gathering methods had 
sufficiently demonstrated their superiority of per- 
formance and their dependability (including im- 
munity to enemy countermeasures) to preclude the 
possibility that optical methods might suddenly as- 
sume critical importance. Consequently intensive 
work on all aspects of the optical range finder pro- 
gram were still receiving substantial Service support 
as OSRD activities were transferred to the Services. 
The most enduring values, however, are probably 
the understanding and the extensive quantitative 
data now available on the design of instruments to fit 
human operators. In the guidance of such investiga- 
tions the author has played a leading role. 

Despite the current and probable future domina- 
tion of radar, much of the work surveyed by this vol- 
ume is of continuing interest provided it is accessible 
to the worker. It is believed that this volume, pre- 
senting an organized abstract of the extensive opera- 
tor-instrument studies and all aspects of optical range 
finders, will enable the officer or investigator con- 
cerned with these areas to gain rapid access to per- 
tinent material which might otherwise be submerged 
by the sheer quantity of material involved. 

H. L. FIazen, 
Chief, Division 7 



PREFACE 


T his monograph is an attempt to describe the ex- 
perimental work performed on the development 
of optical range finders and their operation during 
the war years, by the Fire Control Division of the 
National Defense Research Committee and by other 
agencies so far as is known by the author. The bibli- 
ography of more than 600 titles will indicate the 
enormous amount of work which has been completed 
on this topic. Most of these reports will be found 
in Service files. 

Because of the large amount of material, no 
attempt is made in the present text to give complete 
description of any single experimental study. Rather 
the text shoidd be used as a handbook to indicate to 
a newcomer at a Service desk what has been accom- 
plished and in what sources he may find complete 
and adequate descriptions of individual experiments. 

Early in the development of this Section of the 
Fire Control Division, it was realized that one must 
consider the man-instrument combination in order 
to obtain the best results in operation. Hence re- 


search was carried on to improve the optical ranging 
instruments themselves, to devise new and better 
instruments, to improve methods of operation and 
calibration under field conditions, and also to pro- 
vide methods for selection of those Service personnel 
who would become the best operators, and to provide 
better and more economical methods for the train- 
ing of these selected personnel. Hence certain chap- 
ters will deal almost exclusively with problems of 
physical optics, others with problems of physiological 
and psychological optics and still others with prob- 
lems of psychological selection and training. 

A more complete description of the history of this 
work, its objectives, and of the responsible personnel 
will be found in the Introduction, Chapter 1. Direct- 
ly responsible for the direction of the work of the 
Section was Dr. Thornton C. Fry and later Mr. P. R. 
Bassett, who subsequently replaced Dr. Fry as Chief 
of the Section. 

Samuel W. Fernberger 


R FS l R1C7TD 


ix 


This volume, like the seventy others of the Sum- 
mary Technical Report of NDRC, has been writ- 
ten, edited, and printed under great pressure. 
Inevitably there are errors which have slipped past 
Division readers and proofreaders. There may be 
errors of fact not known at time of printing. The 
author has not been able to follow through his 
writing to the final page proof. 

Please report errors to: 

JOINT RESEARCH AND DEVELOPMENT BOARD 
PROGRAMS DIVISION (STR ERRATA) 

WASHINGTON 25, D. C. 

A master errata sheet will be compiled from these 
reports and sent to recipients of the volume. Your 
help will make this book more useful to other 
readers and will be of great value in preparing any 
revisions. 


CONTENTS 


CHAPTER PACE 

1 Introduction 1 

2 Some Fundamental Studies 5 

3 Field Comparison of Instrument Types 17 

PART I 

ERRORS IN STEREOSCOPIC INSTRUMENTS 

AND THEIR CONTROL 23 

4 Perspective Error 25 

5 Temperature Effects 35 

6 Power and Base Length 54 

7 Calibration of Range Finders 57 

8 Miscellaneous Instrument and Operational Defects ... 63 

PART II 

THE MAN -INSTRUMENT COMBINATION 67 

9 Psycho-Physiological Factors of Operators 69 

10 Relative Position of Reticle and Target and Importance of 

Tracking 82 

1 1 Effects of Haze and of Atmospheric Scattering 94 

12 Miscellaneous Factors of Operation 99 

PART HI 

THE HUMAN OPERATOR 105 

13 Selection of Range Finder Operators 107 

14 Training of Range Finder Operators 133 

PAP^,T IV 

NEW DEVELOPMENTS OE RANGE FINDERS 147 

15 Development of Short-Base Range Finders and Their Ap- 
plication to Ground and Aerial Targets 149 

16 Reticle Design 161 

17 New Instruments 175 

Bibliography 179 

Index 195 

Note: OSRD appointees, project numbers, and con- 
tract numbers are listed at the end of Volume 1. 

^TEvntimin xi 






Chapter 1 

INTRODUCTION 


T his text is not intended to be an encyclopedia on 
ranging instruments or ranging techniques. It is 
an attempt to present, in orderly and systematic form, 
the developments known to Section 7.4, NDRC, of 
the war years 1940 to 1944 in the instrumental and 
operational fields for optical range finders and cer- 
tain related subjects. Its purpose is to orient some 
Service individual who may want information re- 
garding either the trends and developments over the 
entire field or in regard to some specific problem. 
The bibliography will indicate that there have been 
nearly 600 titles issued during this five-year period. 
(Numbers in parentheses in the text refer to items in 
the bibliography.) Most of these reports will be 
found in Service files. This text is merely a systematic 
arrangement of abstracts of these many titles and 
should act as a starting point to guide an individual 
to the pertinent original source or sources for his 
information. AVith changing personnel in Service 
Commands and in Service Offices, it is believed that 
much of this material will be lost and disregarded 
unless some such device as the present text is avail- 
able for orientation. 

The list of titles includes, it is believed, all reports 
which have been submitted to the Fire Control Divi- 
sion of the National Defense Research Committee 
bearing on these subjects. To these have been added 
pertinent titles issued by the Applied Psychology 
Panel, NDRC, and it is believed that this list is com- 
plete. Also will be found titles from various United 
States Service sources and from British sources, 
notably the Armament Research Laboratory 
[A.R.L.]. It is certain that titles from both these 
sources are not complete, but there are listed all 
items readily available to the writer. 

Immediately after the formation of the National 
Defense Research Committee in 1940, the Fire Con- 
trol Section under the chairmanship of Dr. Warren 
\V eaver began a survey of the entire chain of events 
in the fire control system from the obtaining of the 
target position to the firing of the guns. At this time 
they decided that one of the principal sources of 
error was to be found in the optical range or height 
finders which were the standard ranging instruments 
available at that time. They also decided that the 


antiaircraft problem was not only that presenting 
the greatest complication due to extreme ranges and 
to angular rates of the targets but also this was a 
period in the war when improvement of antiaircraft 
fire was essential because of German air superiority 
at that time. A tJieoretical estimate of the effect of 
range finder accinacy on battery performance will be 
found in a Frankford Arsenal Princeton Branch re- 
port. (229) Hence, it was the belief of this Section 
that, if the antiaircraft optical range finder problem 
could be solved, one could apply these results to 
other and simpler situations. 

Dr. Thornton C. Fry was placed in charge of the 
work with optical range finders. Subsequently Dr. 
Harold L. Hazen became Chief of the Fire Control 
Division, and later, upon the withdrawal of Dr. Fry 
from Division membership, P. R. Bassett became 
Chief of the Section in which the range finder activi- 
ties were lodged. The NDRC Fire Control Division 
came to a third important decision— namely, that one 
could not consider a range finder instrument in isola- 
tion but that, to obtain a real insight into the situa- 
tion one must consider the combination of the in- 
strument and its operating personnel in combina- 
tion. This realization of the importance of the man- 
instrument combination very greatly influenced the 
pattern of subsequent research initiated by this Sec- 
tion of NDRC. 

As a result of this survey and of these decisions, 
the Fire Control Division set up an organized pro- 
gram of research to attempt to determine how the 
optical rangefinding instruments could be improved, 
and how various sources of error could be recognized 
and eliminated or controlled, and how one could im- 
prove operating performance by selection of range- 
finder personnel and training them in the use of the 
instrument. To this end, two sorts of projects were 
initiated. Princeton University agreed to set up a 
laboratory at the Antiaircraft Center at Fort Mon- 
roe. Here work was done with actual instruments 
under field conditions, using as operating personnel 
Army Test Observers and students in the Antiaircraft 
Height Finder School. The second type of project 
was initiated at a number of universities and aca- 
demic institutions where experiments were carried 


1 


INTRODUCTION 


out under more exactly controlled laboratory condi- 
tions. Such contracts were made with Brown, Dart- 
mouth, Fatigue Laboratory at Harvard, Howe Lab- 
oratory of Ophthalmology, Ohio State, and Tufts. 
Here problems of various sorts primarily involving 
personnel and the use of range finders were con- 
tinued for many months and some of these contracts 
are still in force and still producing fruitful results. 

After some of the more important sources of error 
were discovered, contracts were made with various 
manufacturing firms for special devices and/or re- 
design of the instruments so that these errors could 
be eliminated or at least alleviated and controlled. 
Such contracts were made with the American Gas 
Association Testing Laboratories, Bausch and Lomb 
Optical Company, Eastman Kodak Company, Fox- 
boro Company, and Keuffel and Esser Company. 
Toward the end of the program each of the three 
optical companies— who are the sole commercial pro- 
ducers of range finders in this country— began com- 
plete redesign of optical range finders on the basis of 
knowledge obtained during these several years of 
experimentation. 

In 1942 it was announced that the Antiaircraft 
School would be moved from Fort Monroe to Camp 
Davis. Inasmuch as much of the work of the group 
had been completed, it was decided to close the con- 
tract with Princeton University under which the 
laboratory at Fort Monroe had been conducted. The 
Princeton group was immediately employed by the 
Frankford Arsenal, and became a Princeton Branch 
of that organization for continued research in fire 
control. This arrangement lasted until July 1944. 
Also in 1942 a National Research Council Committee 
on Service Personnel Selection and Training was 
organized with Dr. John M. Stalnaker, Chairman, 
and Dr. Charles W. Bray, Executive Secretary. The 
interests of this committee covered a much larger 
field than that of fire control. However, arrange- 
ments were made by the Fire Control Division of 
NDRC to transfer the problems of selection and 
training of range finder personnel to this new com- 
mittee. They set up field laboratories for the Army 
at Camp Davis, N. C. and for the Navy at Fort 
Lauderdale, Florida and at the Advanced Fire Con- 
trol School at Washington, D. C. Subsequently the 
work and personnel of the National Research Coun- 
cil Committee was taken over by the Applied Psy- 
chology Panel of NDRC when this was formed in 1943 
under the chairmanship of Dr. Walter S. Hunter. 


The Rangefinder Section of the Fire Control Divi- 
sion was fortunate, throughout its existence, in secur- 
ing the cooperation of highly interested and excel- 
lently informed officers in both Services who aided 
with their advice and assisted in the direction of the 
research. At a very early stage this cooperation took 
the form of frequent— usually monthly— meetings of 
a small group at which the purposes and progress of 
the program were carefully examined. These meet- 
ings were so valuable that they were eventually given 
recognition by formal designation of the officers as 
members of a “Steering Committee” for the project. 
As officers were called away to distant assignments, 
others were named to replace them so that this small 
body— never greater than six at one time— continued 
to exist throughout the life of the study. The officers 
who thus cooperated were: from the Army, Colonel 
W. R. Gerhardt, Colonel G. W. Trichel, Colonel 
G. B. Welch, Lt. Col. A. L. Fuller, and Major R. S. 
Cranmer; from the Navy, Capt. M. E. Murphy, Capt. 
P. E. McDowell, and Comdr. S. S. Ballard. Except for 
the complete cooperation of this group, and of the 
Command of the Antiaircraft Heightfinder School, 
the field experiments at Fort Monroe would have 
been impossible, and the adequate interpretation of 
many of the laboratory studies would have been 
difficult. 

The writer of this report, Samuel W. Fernberger, 
was associated with the project throughout, first as a 
consultant in the psychological factors involved, and 
after September 1941 by direct appointment as Tech- 
nical Aide to the Fire Control Division. 

In the following pages will be found the results 
of all of these experiments in a systematic and orderly 
presentation. Little attempt has been made in regard 
to a critical evaluation by the writer. Instead he has 
been content to let the findings of each experiment 
stand on its own merits but has attempted to inte- 
grate the various studies into a consistent picture. 
Frequently the report of any particular experiment 
has been expressed in the own words of the investi- 
gator. The reader is cautioned that he should con- 
sider the present text only as a book of reference and, 
if he desires detailed information, he should turn to 
the original reports of these various studies. 

The reader may be assisted by an outline of the 
following text and an indication of the philosophy 
and logic underlying the particular structure adopt- 
ed. This is particularly true because of the nature 
and widely varied problems treated in the many 


rR^S TRlCTElA 


INTRODUCTION 


3 


relcrcnces here considered. One will find studies on 
instrumental design, physical optics, physiological 
optics, anatomy, physiology, and psychology. 

Following this introduction, in which are sum- 
marized the point of view and the subsequent con- 
tent, is Chapter 2 in which are outlined certain 
fundamental studies which are basic to the entire 
rangefinding problem. In this are outlined several 
laboratory studies which determined the relative 
acuities which might be expected with different types 
of instruments and fields and also a quantitative 
analysis of the relative importance of the factors 
operative in stereoscopic vision. The third chapter 
outlines the comparative studies made with existing 
instrument types. 

The next five chapters (Part I) deal with the 
existing range finder instruments themselves and 
indicate factors of construction and of errors— and of 
their correction— which are independent of the hu- 
man operators. Hence Chapter 4 deals with perspec- 
tive errors. It includes a general discussion; the use 
of end window stops to reduce this source of error 
and the question of the interocular setting to be 
placed in the instrument. The fifth chapter deals 
with temperature effects and considers such palliative 
methods for the elimination of such errors as air stir- 
ring versus charging the instrument with helium. 
An additional section deals with the development 
of thermally stable instrument parts. Chapter 6 out- 
lines the work on base length and power, while 
Chapter 7 discusses the problem of the calibration of 
range finders. Finally, Chapter 8 deals with a group 
of miscellaneous instrumental and operational de- 
fects which do not naturally fall into any of the 
immediately preceding four chapters, such as penta- 
})rism rotation, filters, leveling, and the like. 

The next division, Part II, which comprises four 
chapters, has to do with the man-instrument combi- 
nation, and involves a discussion of effects or possible 
effects of a supposedly adequate trained operator 
using a perfect instrument. Hence these chapters in- 
clude a discussion of certain psycho-physiological 
factors in the operators and have to do largely with 
the operational aspects irrespective of inherent in- 
strumental errors. Chapter 9 describes the various 
psycho-physiological factors of the operators. It in- 
cludes such problems as fusional limits, fatigue, and 
motivation; the effect of lay-off, of loud sounds, of 
sex differences, and of the administration of drugs, 
changes of posture and the like. Chapter 10 outlines 


the experiments indicating the importance of the 
relative position of reticle and target images and 
hence of the importance of tracking, while Chapter 
11 describes the work on the effects of haze and 
atmospheric scattering. A final short chapter (Chap- 
ter 12) discusses such miscellaneous human opera- 
tional problems as continuous versus bracketing 
contact, the focusing of the eyepieces, the height of 
image adjustment, and the use of the range finder as 
a spotting instrument. 

It will be remembered that the discussion in Part 
II assumes a perfect instrument and an adequate 
operator. The two chapters following (Part III) deal 
with the problem of obtaining such an adequate 
operator. The first of these. Chapter 13, outlines the 
research fundamental to obtaining the best Service 
personnel to be given training. This chapter on 
selection describes the work accomplished on each 
of the anatomical and physiological factors necessary 
for adequate performance on a range finder. In this 
connection considerable space is given to the suc- 
cessful search for a test of stereoscopic acuity which 
would validate with subsequent performance. Some 
space is also given to the development of simple tests 
of emotional stability. After the promising operator 
has been selected, he must be adequately trained. In 
Chapter 14 the problems of training are discussed. 
These include such problems as the assessment of 
performance during training, and the development 
of training methods and of training devices. 

Up to this point in the text, the discussion has 
been concerned with existing range finder instru- 
ments, with their operation, and with the selection 
and training of their crews. The last three chapters 
(Part IV) deal with new developments in the art. 
Chapter 15 describes the development of certain 
short-base range finders and their application to 
ground and aerial targets. A short description is 
added, for the sake of completeness, of work on 
simultaneous stadiametric ranging and tracking. 
Chapter 16 deals with a thoroughgoing study of the 
design of reticles for stereoscopic range finders In 
Chapter 17, finally, is very briefly outlined the de- 
velopment of certain new instruments whose design 
was initiated by NDRC and by other agencies. 

At the end is a bibliography of the reports dis- 
cussed in the text. Some few additional titles are 
included for the sake of completeness. Following 
each title is a number in parentheses which indicates 
the chapter or chapters in which reference to this 


RESTRICTED 


4 


INTRODUCTION 


item is made. 

It is assumed that the reader of this summary will 
have a fundamental knowledge of range finders and 
their operation, such as will be found in the Service 
manuals and in Donald H. Jacobs, Fundamentals of 
Optical Engineering, McGraw-Hill, 1943. In order 
to read critically many of the original reports here 
summarized, it will be necessary for the reader to 
have a knowledge of fundamental statistical pro- 
cedures and the ability to interpret final statistical 


values, as well as, in many cases, a knowledge of 
fundamental physical or physiological optics or of 
experimental or personnel psychology. A readable 
outline of such statistical procedures which does not 
require too extensive a background of pure mathe- 
matics will be found in J. P. Guilford, Fundamental 
Statistics in Psychology and Education, McGraw- 
Hill, 1942. An outline of personnel procedures is by 
A. T. Poffenberger, Principles of Applied Psychol- 
ogy, Appleton-Century, 1942. 


Riel'Ll)^ 


Chapter 2 

SOME FUNDAMENTAL STUDIES 


21 INTRODUCTION 

A NUMBER of new optical arrangements have been 
. suggested for the improvement of the acuity of 
range finding instruments. It would have been both 
expensive and time-consuming to have had each of 
these optical systems built into precision instruments 
and their relative accuracy determined by compara- 
tive field studies. Hence a controlled laboratory ex- 
periment was initiated at the California Institute of 
Technology [CIT] to give insight into this problem. 
This was accomplished by building eight different 
optical systems into a single laboratory instrument 
so that the experimenters could shift very rapidly 
from any one arrangement to any other. 

2 2 REPORTS OF STUDIES 

2-21 Description of Six Experimental 
Arrangements Tested 

These tests are reported by CIT. (187) Six more 
or less untried arrangements were tested in conjunc- 
tion with the two currently in use— namely, fixed 
reticle stereoscopic and simple coincidence with a 
divided field. The other optical arrangements were: 

1. Simple full field coincidence. 

2. Simple coincidence with red and green filters 
respectively in each image path, in the belief that 
color fringes might aid in increasing the accuracy of 
the setting. 

3. Simple coincidence, but the images were flick- 
ered alternately. Thus when out of range one sees first 
the image on one side, and then on the other, with 
this cycle repeated regularly, about five times per 
second, or too slowly for retention of vision to take 
place. When nearly in range with this system, the 
target appears to oscillate slightly to left and right. 
Ranging correctly consists of obtaining a single sta- 
tionary image. 

4. Coincidence strips was another arrangement 
and this was similar to the usual divided field co- 
incidence except that the field was divided into about 
30 equal horizontal parts. These horizontal strips 
were sufficiently narrow to insure that ordinary tar- 
gets will be cut by at least one dividing line, irre- 


spective of the target’s position in the field. This 
then eliminates the necessity of holding the target 
at a particular part of the field as is the case with 
the usual coincidence split field instrument. The 
strips alternately pass only the deviated or the un- 
deviated ray, such that, when out of range, the 
target appears as if in several pieces, the number of 
these depending upon the vertical dimensions of the 
image as compared with the width of the strips. 
Ranging consists of removing the relative horizontal 
displacement of these parts so that the image is seen 
as a complete whole. 

5. Ortho-pseudo is a double stereoscopic arrange- 
ment but without a fixed reticle. Two images of the 
target are seen, one displaced vertically by a fixed 
amount with respect to the other. A ray from either 
telescope enters into the formation of each of the 
images, and they are seen stereoscopically. When out 
of range, the images appear at different depths, since 
the deviated ray of one image falls in a different eye 
from that of the other. Ranging consists in obtaining 
the images in a single plane in depth. The CIT ar- 
rangement differed from that of the Mihalyi instru- 
ment in that the images were not inverted with 
respect to one another, and they were at a fixed ver- 
tical separation independent of position in the field. 

6. The last optical arrangement was called stereo 
strips. In this the field was divided into about 30 
horizontal strips as in the case of coincidence strips 
noted above. However, the target is seen stereoscopic- 
ally so that, when not in range, the parts in adjacent 
strips are displaced in depth with respect to one an- 
other. Ranging consists of obtaining the parts of the 
target in a single plane. There is no fixed reticle in 
this arrangement and, as in the case of coincidence 
strips, ranging is possible in all parts of the field. 
This instrument is described in the text. 

2 2-2 Limitations of Arrangements Tested 

There follows a discussion of the limitations of 
the various arrangements. Theoretically, all of the 
coincidence arrangements and the fixed reticle 
stereoscopic are basically equal in sensitivity. The 
two ortho-pseudo arrangements, on the other hand, 
possess a certain theoretical advantage. Whereas for 


RES I RICTED 


5 


6 


SOME FUNDAMENTAL STUDIES 


the fixed reticle stereo, a unit of deviation by the 
ranging wedges produces a given difference in depth 
between the target and the reticles, the separation in 
depth of the images in the ortho-pseudo arrange- 
ments is just double this amount, since the deviated 
ray takes part in the image formation in both eyes. 

Principles of Ranging 

These investigators point out the following gen- 
eral principles in regard to optical ranging. The 
accuracy of ranging may depend markedly upon the 
size, shape, and attitude of the target. In antiaircraft 
work, the target is of irregular contour, the apparent 
size varied and changing with time. The attitude also 
varies with time. Since full theoretical accuracy can 
be realized only for vertical lines or edges, the di- 
vided field coincidence will suffer to the extent that 
the target fails to present a vertical edge of reason- 
able length at the dividing line. The same is true 
of the coincidence strip arrangement, but if the tar- 
get is large, the observer will have a choice of dividing 
lines. For all the other coincidence arrangements, a 
vertical edge would be available for ranging though 
ordinarily it will be of only limited extent. Stereo- 
scopic methods are comparatively indifferent to gen- 
eral target shape. For the fixed reticle arrangement, 
however, greatest accuracy will result for an extended 
vertical edge favorably placed with respect to the 
reticles. On the other hand, since the vertical separa- 
tion of the two images of ortho-pseudo is fixed, then 
depending upon the size, shape, and attitude of the 
target, the images may partially overlap, with the 
attendant psychological difficulty of reconciling the 
existence of two things in the same place. Again, this 
arrangement will be most sensitive when nearby ver- 
tical edges are available for comparison for both 
ortho-pseudo arrangements. 

Effect of Variables 

True king Errors. A principal limitation of present- 
day range finders when used on rapidly moving tar- 
gets lies in the effect of tracking errors. With the di- 
vided field coincidence, ranging ceases if the elevation 
tracking error is such that the target is not cut by 
the dividing line. Only partial remedy is provided 
by the use of the elevation-tracking-error compensa- 
tion knob, since there will always exist a residual 
vertical motion. For fixed reticle stereoscopic, eleva- 
tion tracking error is especially harmful, since ac- 
curacy declines rapidly with increasing separation of 


target and reticles. Furthermore, should the target 
image coincide with a reticle mark, the psychological 
difficulties referred to above will arise. The new 
arrangements here tested are largely free from the 
effects of tracking error. Simple coincidence, coin- 
cidence with filters or flicker, and ortho-pseudo are 
virtually unaffected, provided the target remains in 
the field of view. For the coincidence and ortho- 
pseudo strips arrangements, ranging is possible in 
all parts of the field, but it is necessary for the eye 
to wander from strip to strip as elevation tracking 
error proceeds. 

Visibility Conditions. The ability to range will 
depend upon the amount of light available according 
to time of day, visibility, and seeing conditions. The 
divided field coincidence, coincidence strips, stereo 
reticle, and ortho-pseudo strips make favorable use 
of all the light available. For ortho-pseudo, however, 
the background of sky or clouds of one image over- 
laps that of the other and the loss of contrast under 
conditions of poor visibility may prove serious. For 
simple coincidence or coincidence with filters or 
flicker, this condition disappears when in range and 
should be less detrimental. It is worth noting that 
coincidence filters depend upon the existence of com- 
parable intensities of light of two different wave 
lengths. If then, the target and its background are 
lacking in the green component, for example, rang- 
ing becomes difficult. However, since the removal of 
the filters might be instantly possible, the system 
could be reduced to simple coincidence. 

Poor seeing conditions should affect all arrange- 
ments about equally, except that the twofold basic 
accuracy of the two ortho-pseudo arrangements will 
be offset by a doubling of this effect. This condition 
would not be marked except for targets near the 
horizon. Poor visibility affects the fixed reticle stereo 
in a peculiar way. This is because the observer must 
compare a hazy image of the target with a clear and 
sharp image of the reticles. Systematic errors are 
likely to result, especially since in practice it is cus- 
tomary to make the initial internal adjustment of the 
instrument by comparing a sharp internal target 
with the sharp and clear reticles. In all other arrange- 
ments, systematic errors of this sort are not to be 
expected since the images, whether for internal 
adjustment or for taking range, are of comparable 
intensities and sharpness. 

Personal and Psychological Factors. Finally, some 
variation among the optical systems is to be expected 


CIT TESTS OF NEW SYSTEMS 


7 


with respect to such matters as speed of learning, ease 
and speed of making range settings, fatigue, and 
psychological preference. In particular when the 
stereo reticle arrangement is out of range, the ob- 
server knows immediately in which direction to turn 
the ranging knob to make the necessary correction. 
The same is true for coincidence filters and divided 
field and for ortho-pseudo. However, for simple co- 
incidence and coincidence with flicker, the appear- 
ance of the images is the same whichever the sign of 
the ranging error, and the observer must rely on trial 
and error knob movements. This not only increases 
the time required for making initial contact, but 
adds to the uncertainty of maintaining contact. The 
strip arrangements, both coincidence and ortho- 
pseudo, do not have this disadvantage, provided the 
observer is able to identify the particular strips being 
used, as would be possible if one set of alternate strips 
differed in color from the other. Such a correlation 
may be difficult in the presence of elevation tracking 
errors. 


2.2.3 Performance Tests of the Optical 
System 

For each of these eight optical systems it was possi- 
ble to view each of three targets of simulated air- 
planes with variations in target size, attitude, and 
rate of apparent change of range. Elevation and 
azimuth tracking errors could be introduced and a 
condition of poor illumination and poor visibility 
or haze. The recording device consisted of a scale 
graduated in units of error [UOE] which was read 
to determine the observer’s zero setting, and a battery 
of 16 clocks which recorded the deviation of the 
ranging wedges from zero under conditions of dy- 
namic operation. The operation was an attempt to 
simulate field conditions with the instrument set to 
an error of about 100 UOE, in a direction unknown 
to the operator, so that he would first have to make 
initial contact and thereafter track the dynamic tar- 
get in range. There were three groups of subjects 
selected in accordance with the criteria set up in the 
Army Field Manual. The first group of 21 men made 
a series of static tests for 4 weeks on the Navy Mark II 
Trainer and then 4 weeks on the testing apparatus. 
A second group of 19 selected men worked only on 
the tester for the 4-week period. The third group 
consisted of six men selected from the other two 


groups who stayed on and made a more exhaustive 
series of observations of the various optical arrange- 
ments of the tester involving both static and dynamic 
runs. They worked six 8-hour days per week for 12 
weeks. 

Preliminary Results 

The results of the preliminary experiments for 
the static mean deviations in UOE of eight runs 
indicates the ortho-pseudo arrangement gave the best 
results, with coincidence divided field and strips, 
and stereo reticle not very different. Simple coinci- 
dence, coincidence with filters, and flicker are the 
worst arrangements when the field is viewed either 
monocularly or binocularly. The averages for the 
last four runs, presumably after some practice, do 
not substantially change the rank order although the 
magnitude of the error is reduced for every one of 
the arrangements. The time required by each ob- 
server to complete ten settings was recorded for each 
arrangement for the last four runs. Averaged for 
all observers, these times varied from 7 to 10 minutes 
except for the coincidence flicker which required 
13 minutes. The observers complained that ortho- 
pseudo strips and especially coincidence flicker re- 
quired excessive concentration, and also reported 
eyestrain. 

In another experiment 15 dynamic runs were per- 
formed by the six observers of the third group. Sim- 
ple coincidence, and coincidence with filters and 
flicker were performed with the binocular arrange- 
ment only because it was found superior or equal 
to monocular observation in the previous experi- 
ment. Divided field coincidence was omitted because 
of its similarity to coincidence strips. Readings were 
taken both without and with tracking errors in ele- 
vation and azimuth, with poor visibility, and irregu- 
lar dynamic change as variables. 

Effects of Variables 

A study of the results indicates the harmful effect 
of tracking errors for the stereo reticle, coincidence, 
and ortho-pseudo strips is quite evident. On the 
other hand, poor visibility affects all arrangements 
rather uniformly. The run with irregular dynamic 
change indicated that familiarity with the pattern of 
change did not unduly affect the results of the other 
runs. It is evident that both coincidence and ortho- 
pseudo strips suffer excessively for the targets with 
unfavorable attitude under tracking error. The rank 


\ RESI RICTr.n 


8 


SOME FUNDAMENTAL STUDIES 


order of the best values from smallest to largest errors 
are ortho-pseudo (1.9 UOE); stereo reticles (2.3); 
ortho-pseudo strips (2.5); coincidence strips (2.9); 
coincidence filters (3.5); coincidence flicker (4.0); 
and, finally, simple coincidence (5.0 UOE). The rank 
order for the mean for all conditions is not very 
different from that of the best values, being: ortho- 
pseudo (2.5 UOE); ortho-pseudo strips (4.1); stereo 
strips (4.9); and simple coincidence (6.3 UOE). 

Because the ortho-pseudo arrangement had a 
lower dynamic mean deviation for every one of the 
tests, further experiments were undertaken. These 
tests were made with four of the six men of the third 
group who were now seasoned, trained observers. 
Only the ortho-pseudo and stereo reticle patterns 
were used for this comparison, with coincidence 
divided field introduced as a check. Various combi- 
nations of variables of tracking errors, poor visibility, 
poor illumination, and irregular dynamic change in 
range were introduced. In these tests the ortho- 
pseudo arrangement was consistently superior to 
stereo reticles. The grand means were 0.70 UOE for 
stereo reticles and 0.46 UOE for ortho-pseudo for all 
conditions. The grand mean for coincidence divided 
field was 1.2 UOE even though no results were taken 
with the two most difficult targets with tracking 
errors introduced. An analysis of the results suggest 
that the size and shape of the target are of importance 
only in the case of the split field coincidence and this 
may be a partial explanation for the poor showing 
of this arrangement. 

In a consideration of the dynamic mean deviations 
the results indicate that the two stereoscopic arrange- 
ments are not markedly dependent upon the size and 
attitude of the target. For the basic runs, that is with- 
out any variables introduced, the errors are 20 per 
cent larger for stereo reticles than for ortho-pseudo. 
Tracking error has little effect on ortho-pseudo while 
the detrimental effect on the stereo reticle pattern 
makes the error increase 50 per cent over the basic 
one. For this arrangement, elevation and azimuth 
tracking errors were equally harmful. Poor visibility 
affected ortho-pseudo rather seriously but, under the 
conditions of the experiment, no worse than stereo 
reticles. Poor illumination proved to be only a lim- 
ited handicap for either arrangement. 

Results with Inexperienced Observers 

In a subsequent experiment, CIT made an in- 
vestigation of the relative merits of certain range 


finder optical systems when used by inexperienced 
observers. (188) The application was for short-based 
range finders to be required in great numbers and 
hence with a simple optical system. Accordingly, 
only simple coincidence, coincidence with filters, and 
stereo reticles were used in the tests. Because the 
range finder would be operated by comparatively 
inexperienced observers, the tests were planned to 
emphasize the learning aspect. Because the applica- 
tion envisaged rapid motion of the target and of the 
platform of the instrument, the effects of tracking 
error were given particular attention. The same in- 
strument was used as in the former experiments. The 
subjects were 30 men chosen by selection with visual 
tests. These were divided into six groups of 5 men 
each to make random the order in which the runs 
were taken with the different optical arrangements. 
The results show the greater inherent sensitivity of 
stereo reticles as indicated by the static mean devia- 
tions. The curve for this optical system is relatively 
free from learning, dropping from slightly less than 
2 UOE error in the first session and reaching a 
plateau at slightly more than 1 UOE by the third 
session. The curve for coincidence filters is consist- 
ently below that for simple coincidence for all five 
sessions of the experiment but only one value in 
either curve is below 3 UOE when this value is 
reached by the second session for coincidence filters. 
Tracking error adversely affects all three arrange- 
ments, stereo reticle being rather more affected than 
either of the coincidence arrangements. 

When one considers the values of the dynamic 
deviations of the mean, stereo reticles no longer ap- 
pear markedly superior. This is an indication that 
this arrangement is more subject to systematic errors 
than the coincidence patterns. One observer had 
extreme difficulty with stereo reticles under tracking 
error, being for the most part completely out of con- 
tact. His performance with this arrangement was 
quite normal without tracking error. From these 
results the investigators make the following conclu- 
sions. At the beginning of this program it was feared 
that stereo reticles might be inadequate in two re- 
spects; first, that it would take too long for an in- 
experienced observer to master it, and, secondly, that 
it would suffer unduly from large tracking errors. It 
now appears that stereo reticles are superior to either 
simple coincidence or coincidence with filters. Even 
at the beginning of the trials, it shows smaller values 
for the significant numbers. The values for the final 


MARK 40 TESTS AND NEW OPTICAL SYSTEM TESTS 


9 


performances of stereo reticles ran from 30 to 70 per 
cent higher than comparable figures obtained in 
previous tests for seasoned observers. Also, it is likely 
that an occasional observer will fail utterly with 
stereo reticle under tracking error. 

These two CIT reports are presented as support- 
ing data to a Report to the Services issued by the 
Fire Control Division of NDRC. (27) The results are 
summarized here briefly but no recommendations 
are made to the Services. 


2.2.4 Performance of the Mark 40 
Range Finder 

When Modified by Color Differentiation 
AND Flicker 

In regard to several of these matters, a Bausch and 
Lomb Optical Company report contributes further 
information. (115) The report is divided into two 
parts. The first deals with the performance of the 
Navy Mark 40 Range Finder modified by the inclu- 
sion of color differentiation of the two target images 
and the inclusion of intermittent exposure of the 
target images by a flicker technique, either singly or 
in combination. It was found that when the images 
were interchanged about ten times per second at the 
eyepiece by means of a Polaroid system, precision of 
ranging improved from ±247 UOE to ±165 UOE 
for the average of four observers. When the images 
were differentially colored by means of red and green 
filters over the respective end-windows, the precision 
of two new observers with normal color vision im- 
proved respectively from ±2.89 to ±2.12 UOE. 
These same observers had, however, been previously 
improved to a greater extent than this by the flicker 
feature from ±2.89 to ±1.86 UOE. When both the 
flicker and color accessories were in use, one new 
observer improved in precision from ±3.31 to ±2.35 
UOE. Two other observers obtained poorer preci- 
sion with the plain instrument. One observer who 
was partially color blind was not significantly af- 
fected in his precision by the addition of the color 
feature, whether or not flicker was also present. Also 
a particular weather condition may affect the flicker 
and flickerless operation differently. Heat waves or 
atmospheric boil appears to reduce precision less 
when flicker is present than when it is absent. These 
results confirm the previously reported results from 
California Institute of Technology in emphasizing 


the increased precision when flicker is introduced. 
They do not confirm the finding of increased color 
alone, except when the observers were relatively un- 
trained. Hence for flicker the gain is very similar for 
the Bausch and Lomb and CIT experiments when 
expressed in per cent— the values being 33 and 25 per 
cent respectively. The results for color alone are 
quite different, however, being 24 per cent for Bausch 
and Lomb and 5 per cent for CIT. 

When Target is Off-Center in Visual Field 

The second part of this report has to do with rang- 
ing with the target off center in the visual field. It 
was found that no significant change in accuracy 
occurs with the Mark 40 Range Finder when the 
target is placed as much as 10 degrees of apparent 
field angle below, above, or to either side of the 
center. Precision falls off no more than 0.5 UOE in 
any of these meridians and it is believed that these 
losses probably have a largely optical and not essen- 
tially perceptual basis. Expert stereoscopic range 
takers perform with great inaccuracy and with poor 
or extremely poor precision in attempting, without 
special training, to range on a target 6.5 to 10 degrees 
of apparent field angle below the central reticle 
marks of stereo range finders. Three out of four in- 
expert stereo range takers learned in a few days to 
range with acceptable precision on targets as much 
as 5 degrees above or below the fiducial mark. Only 
one of the three attained acceptable accuracy. The 
inaccuracies of the other two were systematic and 
apparently due to the effects of torsional movements 
of the eyeballs, normal in those observers, upon their 
stereoscopic vision. For them, the perceptual plane 
of the reticle was a sloped or warped surface. The 
fourth observer, allowed the same amount of prac- 
tice as the other three, could not learn to range off 
center with either acceptable precision or accuracy. 

2.2.5 Optical Systems in Relation to 
Field Types 

A report from the Princeton Branch of the Frank- 
ford Arsenal describes modifications to the M6 
Stereoscopic Trainer enabling it to be used for ex- 
periments to determine comparable results from 
different optical arrangements rapidly and easily. 
(262) This modification would provide ortho-pseudo- 
scopic, stereoscopic reticles, ortho-pseudoscopic stereo 


[restrict^ 


10 


SOME FUNDAMENTAL STUDIES 


coincidence, split field coincidence, both erect and 
inverted fields viewed binocularly and monocularly. 
Various targets may be introduced including vecto- 
graphs and Kodachrome stereo-pairs, and even actual 
external targets may be used for ranging. These 
modifications are described in detail in the report. 

This instrument was used in a comparative study 
of types of field which are reported in another study. 
(241) Eight field types were used: superimposed co- 
incidence, erect coincidence, invert coincidence, 
ortho-motion, invert coincidence pseudo motion, 
invert ortho-pseudo, erect ortho-pseudo, ortho stereo 
with reticles, and pseudo stereo with reticles. Three 
trained observers ranged on three targets which were 
vectographs of a telephone pole, a tank half hull 
down, and a tank head on. Because of the nature of 
the targets no measure of true range could be deter- 
mined. The results indicate that, for targets of this 
sort, superimposed coincidence is definitely inferior 
to all of the other field types in both precision and 
consistency of ranges. Erect ortho-pseudo appears 
inferior in precision, though its consistency is good. 
Pseudo-stereo with reticles is inferior in consistency. 

In another report by the Frankford Arsenal, Prince- 
ton Branch, is reported a comparative study of invert- 
foreground and invert-sky in both monocular split- 
field coincidence and ortho-pseudo stereoscopic 
types of field. (242) Averaged over all observers, for 
simulated ground targets, no significant differences 
in consistency or precision were found between co- 
incidence and ortho-pseudo presentation or between 
invert-sky and invert-foreground fields. Individual 
observers showed marked differences, however, in 
performance with respect to type of presentation. 
One observer did well on ortho-pseudo and not so 
well on coincidence, while another performed well 
with coincidence and poorly with the ortho-pseudo 
presentation. No significant differences between ob- 
servers were shown for type of inversion. 

Still another comparative study of types of field is 
reported by the Princeton Branch. (261) Four types 
of field were compared; invert coincidence, invert 
ortho-pseudo, stereo with reticles, and superimposed 
coincidence. In the invert fields, the upper half field 
was erect and the lower half inverted because the 
previous work had shown that the results with invert 
fields are unaffected by the choice of upper or lower 
half field for inversion. Six vectograph photographs 
of ground targets were used and these were ranged 
on by two experienced observers. The results indi- 


cate that, in regard to precision, superposed coinci- 
dence gives the largest spread for both observers and 
for all targets. As between the remaining fields, the 
results differ with the observer. One performed best 
on stereo while the other’s stereo performance was 
somewhat worse than either of the other fields. As 
between invert coincidence and ortho-pseudo there 
was no consistent superiority for either field with 
either observer. As regards consistency, the results 
show clearly that superposed coincidence is definitely 
more variable than the other fields. Between the 
remaining fields, the case is not so clear. In general 
one observer did best on stereo reticle while the 
other showed no consistent superiority with either 
ortho-pseudo or stereo reticle. 

2.2.6 Precision, Consistency and Accuracy 
of Visual Range Observations 

A fundamental study of the precision, consistency, 
and accuracy of visual range observations, both of 
the stereoscopic and vernier types, is reported by the 
Harvard Fatigue Laboratory (267) and is attached as 
supporting data to a Report to the Services issued by 
the Fire Control Division of NDRC. (5) This report to 
the Services merely passes on the Harvard report for 
their information and makes no recommendations. 

The Harvard report points out that a systematic 
study of the laws of stereoscopic acuity may be ap- 
proached from the standpoint of commonplace 
stereo vision or of telescopic stereo vision. Each mode 
of approach possesses its own peculiar advantages 
and disadvantages. The major advantage of using 
commonplace vision (“free space”) at the beginning 
of a general investigation lies essentially in the fact 
that an almost unlimited number of ocular variables 
may be studied with a relatively small expenditure of 
time and effort. The chief disadvantage of this ap- 
proach resides in the fact that one is sometimes un- 
certain whether the commonplace results apply to 
telescopic vision. 

There is one problem, however, basic to both 
methods of approach, which may be studied with 
profit under commonplace circumstances. One may 
undertake to discover the psychophysical conditions, 
general or specific, under which stereoscopic vision 
is most precise (i.e., minimal variation for a single 
set of observations); maximally consistent (i.e., the 
observed range does not vary from day to day any 


[ RES TRICT^^ 


HARVARD STUDIES ON STEREOSCOPIC ACUITY 


1 


more than is expected by random sampling from a 
homogeneous universe); and sufficiently accurate 
(i.e., the departure of the observed range from true 
range is acceptably small). The need for this kind of 
undertaking arose from the fact that the funda- 
mental laws governing these three measures of stereo- 
scopic acuity, any one of which can be varied inde- 
pendently from the other two in the laboratory, were 
almost completely unknown. 

Therefore the Harvard experiment sought to de- 
termine the general rules which govern the influence 
on stereo acuity of alterations in observational cir- 
cumstances as they have been systematically varied 
under a variety of controlled conditions and also, by 
further investigations, to isolate some of the con- 
ditions producing optimal effects. Experiments were 
carried on concerning the performance of normal 
observers under commonplace binocular stereo con- 
ditions and under monocular vernier observation. In 
most of the experiments, enough measurements were 
obtained to be statistically reliable. Some of the more 
reliable results show conflicting indications. Others 
have proved difficult to understand. Still others seem 
to have no practical application. At best, the net 
result is one of considerable complexity. From be- 
ginning to end, moreover, it was necessary to conduct 
the investigations at an empirical rather than at a 
strictly rational level. At the empirical level, it is 
believed that an advance has been made and that 
the results are sound enough so that the investigators 
could turn profitably to the study of stereo vision 
under telescopic conditions. For example, the in- 
vestigators feel certain that they have succeeded in 
the attempt to specify some of the general conditions 
under which the precision, consistency, and accuracy 
of stereo range observations will — from one time to 
another during any given day, and from one day to 
the next, over a period of several months — remain 
essentially constant at a value equal to 1 or 2 seconds 
of arc. 

Conclusions 

The general conclusions deriving from these ex- 
periments, detailed below are: 

1. Coincidence acuity increases with target dis- 
tance, and approaches a limit as the eye approaches 
a relaxed state of accommodation. This result con- 
firms the Aubert-Foerster effect. 

2. Stereo acuity is a variable quantity. For the same 
target reticle assembly, the so-called “minimal” 


parallax angle can be made to vary systematically 
with the convergence and accommodation of the 
eyes, with the apparent distance of the target, with 
the vertical attitude of the eyes, with the mode of 
fixation, with observational criteria, and in minor 
ways with a number of other factors. The smallness 
of an observer’s “minimal” parallax angle, therefore, 
depends on conditions prevailing in the observer. 

3. Nonetheless, stereo acuity is good and depend- 
able if special psychophysical conditions are prop- 
erly controlled. If the converged distance equals the 
accommodated distance equals the apparent distance 
equals the target distance (remotely situated), then 
the precision, consistency, and accuracy of common- 
place stereo range observations are of the order of 
approximately one second of arc. When these con- 
ditions are violated, the observations suffer a reduc- 
tion in precision and consistency. The amount of 
reduction depends on the conditions violated and 
the magnitude of the violation. The reduction may 
be 100-fold or more. In other words, (a) when the 
conditions are violated, stereo acuity is low. Pre- 
cision usually exceeds consistency. Consistency may 
be of the order of 1 or 2 minutes of arc. (b) When the 
conditions are all fulfilled, stereo acuity is high. Con- 
sistency exceeds precision. Accuracy may exceed con- 
sistency. Precision, consistency, and accuracy all 
equal about 1 to 2 seconds of arc. Hence the general 
final conclusion may be drawn that the accuracy of 
stereo range measurements can be controlled. 

Details of Experiments 

In the first experiment, on which these conclusions 
are based, the precision, consistency, and accuracy of 
stereo range observations were measured at each of 
a number of target distances. The converged, ac- 
commodated, apparent, and target distances were 
kept equal throughout. The experiment sought to 
challenge the prevailing notion that the “minimal” 
visual angle for stereo vision remains constant when 
the distance is varied. It was found that precision 
and consistency increased as the target distance was 
increased. This finding, which is the stereoscopic 
counterpart of the Aubert-Foerster effect for ordi- 
nary visual acuity, has definite implications for tele- 
scopic stereo range finders. For high stereo-acuity 
(1 to 2 seconds of arc), the accommodated distance 
of the target and reticle in apparent visual space 
should exceed 20 to 30 feet. It was also observed that 
the inaccuracy of the range measurements decreased 


12 


SOME FUNDAMENTAL STUDIES 


as the target distance increased. The instruments 
devised for this and subsequent experiments are 
described in the text. A number of very highly 
trained observers were used throughout. 

Effect of Distance. The fact that acuity was high 
for remote distances raised the question concerning 
the possibility of securing even higher values at still 
greater distances. Experimental conditions were ar- 
ranged which allowed the observers to demonstrate 
their ability when the converged distance was equal 
to, or nearly equal to, infinity. Under these conditions 
the mean variation of stereo observations was found 
to be of the order of 1 to 3 seconds of arc at distances 
greater than 10 meters when the proper conditions 
were fulfilled. The mean variation of the means of 
these stereo observations was less than the mean of 
the mean variations. When the conditions were vio- 
lated, on the other hand, the average of the mean 
variation of the stereo observations was increased to 
about 6 seconds of arc. The mean variation of the 
means of the observations, furthermore, was greater 
than the mean of the mean variations. 

This kind of situation is not what one expects in 
observations behaving normally in accordance with 
the law of error. Rather it is what one might expect 
from results obtained by sampling in a biased man- 
ner from a single homogeneous population, or by 
sampling in a random manner from one universe at 
one time and, in the same manner, from each of a 
number of others at other times. If biased sampling 
represents the situation, then the fact that the aver- 
age of the mean variations is smaller than the mean 
variation of the means should be expected, and the 
latter index is the measure to use in general for pre- 
dicting the variability of the stereo range observa- 
tions. If different universes have been sampled from 
one time to another, a similar admonition is in order. 
In either event, the same experimental solution is 
indicated. One population (or bias) should, if pos- 
sible, be consistently chosen by the observer. 

Inasmuch as these were highly trained subjects, 
there can be little doubt that each observer did his 
best under both sets of circumstances. The investi- 
gators believe that the principal reason why the mean 
variation of the means was greater than the mean 
of the mean variations, when the eyes were verged to 
infinity, is that normal eyes invariably accommodate 
to the converged distance and thus the retinal images 
of the target and reticle suffered a loss in definition. 
Such a situation produces a visual conflict. If the 


observer verges exactly to the target and reticle, he 
soon recognizes by trial and error experience that 
the definition can be improved by increasing his 
convergence. If the convergence is increased too 
much, however, double vision results. This abnormal 
state of affairs sets up a fluctuating tendency which 
could increase the mean variation of the averages 
of stereo observations not only because it provides 
an opportunity for different vergence-accommoda- 
tion biases but also because, from one set of observa- 
tions to the next, it provides an excellent oppor- 
tunity for the occurrence of varying degrees of cyclo- 
torsion and these different degrees of cyclotorsion 
will practically always produce differential changes 
in the apparent distance of the target with respect to 
the apparent distance of the reticle when either one 
is located directly above the other in the visual field. 

Effect of Attitude. The next experiment was aimed 
at the control of attitude and its relation to accuracy. 
Three different attitudes of the eye were used: reticle 
centrally fixated, fixation midway between recticle and 
target, and target centrally fixated. The results show 
that the accuracy or range observations can be con- 
trolled, in absolute amount, by the proper direction 
of the observer’s eyes. The effect itself can be under- 
stood simply in terms of retinal disparities produced 
by excyclotorsion (the summits of the eye recede 
from each other as the eyes turn imaginary axes 
through the foveas and the point of fixation) during 
each fixation. When the eyes turn about these axes, 
a constant angular turn produces a binocular retinal 
disparity which increases with the vertical distance 
from the foveas. Thus, as the observer fixated mid- 
way between the reticle and the target, and made his 
stereo setting accordingly, his results evidenced only 
a slight negative inaccuracy. However, as he fixated 
the reticle, then the target was seen relatively more 
remote in apparent space due to the increased dis- 
parity produced by the constant amount of excyclo- 
torsion. To execute his task properly, therefore, the 
observer reduced the apparent position of the target, 
and his observed readings fell short of the true range. 
The same sort of reasoning may be applied to the 
situation in which the target was fixated, with the 
same result. 

This effect can occur with the reticles now used in 
Army and Navy instruments. That it actually does 
occur is proved by reports from stereoscopic observers 
in the Services that they occasionally see the diagonal 
reticle marks in reversed perspective. 


c 


RESTRICTED 




HARVARD STUDIES ON STEREOSCOPIC ACUITY 


13 


Effect of Stereo Acuity and Vergence Change. The 
next experiment was aimed at a study of stereo acuity 
and vergence changes. As an observer fixates the tar- 
get or the reticle in a stereoscopic range finder, the 
vergence of his eyes alters from time to time within 
the limits of binocular fusion and the experiment 
sought to determine the effect of such changes on 
stereo acuity. The physical distances of the targets 
ranged from 300 to 2,400 cm. The vergence was 
changed by means of a pair of crossed prisms placed 
directly in front of the eyes of the observer and with 
settings changing the vergence from -|-2 degrees 18 
minutes to —2 degrees 18 minutes of arc. Such ver- 
gence changes would occasion changes in converged, 
apparent, and accommodated distance. 

The results indicate that the precision decreases as 
the converged distance departs more and more from 
the target distance. The rate of this decrease in pre- 
cision is not the same for all target distances. At the 
most proximal distance (of 300 cm) the rate of 
change in variability per unit of change in vergence 
is greatest. In other words, for short apparent dis- 
tances, the precision falls off most rapidly as the con- 
verged distance is either increased or decreased. At 
the most remote target distance (of 2,400 cm) ver- 
gence changes produce very little effect on the magni- 
tude of precision. In regard to consistency, for the 
most proximal distance there is a rapid decrease as 
the change of the vergence angle is increased. The 
trends showing the effect of vergence changes upon 
the accuracy of range observations are not regular. 
For remote distances, where the inaccuracy is very 
small, changes in the converged distance produce 
little effect on the magnitude of the measurements. 
At the more proximal distances, however, the in- 
accuracy varies considerably when the converged 
distance is no longer equal to the target distance. 

Effect of Coincidence or Vernier Acuity. Still an- 
other experiment outlined in this Harvard report 
was concerned with coincidence or vernier acuity. 
The field of view, target, reticle, distances, and the 
adjustments were the same as those used in the 
binocular experiments but light was occluded from 
the left eye by a stop. In these experiments, the con- 
sistency exceeded the precision in all cases but one. 
At the shortest distance, the inaccuracy was large- 
varying from about —35.7 to about -|-150 seconds of 
arc for three observers. At the longest range, the in- 
accuracy was small, varying from —0.03 to -fO.8 sec- 
onds of arc. 


A comparison of the binocular and monocular 
results yields the following. The difference between 
the average mean variations is positive and large for 
short ranges. As the range is increased, the difference 
decreases up to about 20 to 30 feet. The difference 
between monocular and binocular precision is in- 
significant at more remote distances. The consistency 
differences behave in essentially the same manner as 
those for precision. The monocular accuracy results 
for two of the three observers are all positive. Thus, 
the average for the observed range is slightly greater 
than the true range at each of the six distances. For 
the third observer, most of the departures are nega- 
tive. For binocular vision, these errors are consider- 
ably smaller at proximal distances. There is little 
difference, however, between the binocular and 
monocular results when the perceived distance of 
the target is great. The results of these experiments 
demonstrate that the precision, consistency, and ac- 
curacy of range measurements for binocular regard 
are better than those for monocular vision when the 
perceived distance of the target and reticle are short. 
When the perceived distance is great, the two modes 
of observation are about equally good. 

Just Noticeable Differences 

In the second half of this report, the investigators 
discuss the relation of just noticeable differences as 
compared with the equality settings described in the 
first section. These were obtained under both binocu- 
lar and monocular viewing and for both proximal 
and remote distances. They are judgments of when 
the target appeared just noticeably nearer or further 
from the reticle. The reciprocal of a just noticeable 
difference [j.n.d.] is usually designated as a measure 
of differential sensitivity of the observer. In their 
earlier work the just noticeable differences and the 
precision measure of range estimates were believed 
to be of about the same order of magnitude for com- 
parable conditions. They therefore argued that these 
two measures are probably determined by the same 
underlying physiological mechanism. The section 
under discussion presents data which indicates that 
this hypothesis is not consistent with all the facts. 
For binocular vision the magnitude of differential 
sensitivity is much less than the magnitude of the 
precision of range estimates for practically all dis- 
tances. The trends are the same, however, as the 
j.n.d. diminishes as the target distance increases. The 
difference between corresponding proximal and re- 


_^STRICTFnl 


14 


SOME FUNDAMENTAL STUDIES 


mote j.n.d. for any given observer, moreover, is least 
at the greatest distance. At more proximal distances, 
the corresponding proximal and relatively more re- 
mote values in some instances differ greatly, although 
in a number of other instances the two values are 
almost equal. The sum of the proximal and remote 
j.n.d. for any target distance has been known histori- 
cally in psychophysics as the interval of uncertainty. 
In angular units, the magnitude of the j.n.d. in dis- 
tance becomes progressively smaller as the target 
distance increases. The data for j.n.d. for monocular 
sensitivity also show, in all cases, the tendency to 
decrease with distance, and the rate of change is most 
rapid initially and then more attenuated. While the 
difference between corresponding proximal and re- 
mote values is often very great for any given nearer 
distance, for the more remote distances the differ- 
ences are very small. 

A comparison of the results regarding distance 
sensitivity as obtained under binocular and monocu- 
lar conditions of observation reveals several points. 
First of all, the j.n.d. in both cases exhibits a ten- 
dency to decrease as a function of distance. The rate 
of change in this respect is greater for the monocular 
data, owing to the fact that the j.n.d. for proximal 
distances is much greater for monocular than for 
binocular observation, while the values for both con- 
ditions of observation at the greatest distance are of 
the same order of magnitude. The differences be- 
tween proximal and remote values for a given ob- 
server and for a given distance are also greater for 
monocular than for binocular observation. 

Magnification and Base Length 
Stereoscopic Instruments 

A final chapter of this Harvard report deals with 
problems of magnification and base length of stereo- 
scopic instruments. In the magnification experiments 
large Zeiss terrestrial binoculars were employed us- 
ing two magnifications of 0.8x and 12x which are in 
the ratio of 1 to 15. From the standpoint of geometri- 
cal optics, a 15-fold reduction in the precision of the 
range measurements is to be expected, if one assumes 
that the observer’s stereo acuity remains constant for 
these two sets of conditions. If, however, one takes 
into account the fact that an increased magnification 
reduces the accommodated distance of all objects in 
the field of view, then, since the converged distance 
remains unchanged, when the target and reticle are 
properly centered, one would actually expect a de- 


crease in stereo acuity with an increase in magnifica- 
tion. The results reported showed only a 3-fold in- 
crease in the precision and in the accuracy of the 
actual range settings when the magnification was 
increased 15 times. The observers’ stereo acuity, 
therefore, was reduced to one-fifth of its original 
value, in seconds of arc, as the magnification of the 
target and reticle was increased. The advantage 
afforded by magnification was evidently opposed by 
some compensating factor— probably the accompany- 
ing reduction in the distance of the target as seen in 
telescopic space. 

Study of Stereo Acuity and 
Telescopic Vision 

Conclusions 

Another experiment is reported in an attempt to 
obtain a rough estimate concerning the effect of 
magnifying the parallax angle per se, that is without 
magnifying the retinal images of the target and 
reticle. This was performed with a telestereoscope 
possessing a variable base. This instrument has the 
effect of greatly increasing the interpupillary dis- 
tance of the observer and hence magnifies the paral- 
lax angle. It was found that the observers’ stereo 
acuity was reduced as the base was increased. Stereo 
acuity for these observations was thus greater for the 
observer when he used the normal interpupillary dis- 
tance than when he viewed the field with the aid of 
an enlarged base. Stereo precision was found to de- 
crease continuously as the length of the base was 
increased. Hence as the range finder is increased, 
the converged distance is greatly diminished, as com- 
pared with the distance to which the eyes of the ob- 
server ought to be accommodated if the target and 
reticle images in his retinas are to be sharply defined. 
Still another factor which may be operating effec- 
tively to offset the optical advantages afforded by the 
large base is the factor of apparent size. 

This group of experiments support the conclu- 
sion that stereo acuity is both good and dependable 
under the special conditions of the converged dis- 
tance, the accommodated distance, the apparent dis- 
tance, and the target distance all being equal. In this 
case, the experiments with commonplace vision in- 
dicate that the accuracy, precision, and consistency 
of stereo range observations are of the order of 1 or 
2 seconds of arc. Too much emphasis cannot be 


STRICT EPy 


HARVARD STUDIES ON STEREOSCOPIC ACUITY 


15 


placed on the maintenance of this 4-fold equality, 
because when these conditions are violated, in any 
way whatsoever, the observations suffer a reduction 
in precision and consistency up to 100-fold or more. 
The amount of reduction will depend on the condi- 
tions violated and the magnitude of the violation. 

These laboratory studies of stereoscopic acuity 
were continued at Harvard University for both un- 
aided and telescopic vision. (283) Stereo acuity is 
ordinarily measured in angular units at the observ- 
er’s eyes and is assumed to be independent of mag- 
nification, base length, and range when the eyes are 
used in conjunction with an optical instrument. If 
this measure is valid and the assumption correct in 
general, then the angular error of a stereo range 
finder, as measured at the instrument, varies in- 
versely with magnification and base length, but is 
independent of range. This expectation issues di- 
rectly from the well-known equation relating the 
angular range error to range, magnification, and 
base length. 

The Harvard investigations were made under a 
variety of field and laboratory conditions and, in 
general, failed to bear out this expectation. Stereo 
acuity, as measured in angular units at the eyes of 
the observer, was not found to be independent of 
magnification and range. Instead, the results indi- 
cated that stereo acuity: (1) became poorer as the 
magnification increased at any particular range; 
(2) improved as the range increased for any particu- 
lar magnification; and (3) was about 100 times better 
than is conventionally supposed. 

An analysis of the conditions under which these 
results were obtained indicated that three different 
distance cues were available to the observers during 
the experiments. These cues were isolated physically, 
and the strength of the cues measured with reference 
to their effect on the observer’s depth perception: 
(1) when all three cues were available; (2) available 
in pairs; and (3) available one at a time. All of these 
conditions were investigated in free space and as 
modified by telescopes of various powers. In consid- 
ering the results of this analysis it was found con- 
venient, and at times necessary, to express range 
errors in linear units. Most of the data, therefore, is 
expressed in yards or in per cent yards. 

The results may be summarized as follows. A series 
of field experiments was performed in order to de- 
termine the relation between stereo acuity and mag- 
nification. The experiments were carried out both 


on land against a terrestrial background and on the 
water against a fairly uniform background of sea 
and sky. Measurements were obtained for a number 
of different magnifications, varying in power from 
lx (the unaided eye) to 40x, and for different ranges, 
varying from 50 to 6,400 yards or more than 3 miles. 
For each of the three highly trained observers who 
served in all the experiments, the results were essen- 
tially the same, and they present the following indi- 
cations. 

The relation found between magnification and 
stereo acuity indicated that the angular error at the 
eye was not constant, but increased in direct propor- 
tion to the increase in magnifying power. Expressed 
in per cent units (=100xAR/R) the error was con- 
stant and independent of magnifying power. The 
relation found between range and stereo acuity in- 
dicated that the range error in per cent was not 
proportional to the range, as conventional theory 
assumes. Instead the per cent linear error was found 
to be nearly, though not exactly, independent of 
range. Moreover, the per cent error of the observa- 
tions was unexpectedly low. For any of the ranges 
or magnifying powers employed, the average mean 
variation or the mean variation of the average ad- 
justments were always less than 1 per cent. Even the 
averages for insensitivity did not exceed 5 per cent 
at any range and, for the more remote distances, 
they also fell below 1 per cent. 

Details of Experiments 

The results of the field experiments led to two 
lines of further experimentation in the laboratory 
by the Harvard group: (I) a verification and under- 
standing of the relation between magnification and 
stereo acuity, and (2) an analysis of the properties 
of the visual end point which gave such high preci- 
sion, consistency, and sensitivity. 

To obtain the laboratory results, the original 
study was repeated under conditions which approxi- 
mated as nearly as possible the fundamental condi- 
tions of the field experiments, although the ranges 
used were necessarily reduced in the laboratory to 
20 to 60 yards. The angular size of the target and 
reticle were not kept constant, but were allowed to 
vary naturally with range and magnification, as in 
the previous experiments. The laboratory results 
confirmed the relations indicated above. The angu- 
lar error at the eye was found to increase in direct 
proportion to the increase in magnification. Ex- 


RFSTRICTED 


16 


SOME FUNDAMENTAL STUDIES 


pressed in terms of per cent error, the error of the 
observations was independent of magnifying power. 
For the three distances available in the laboratory 
experiment, the results again indicated that the 
range error, in yards, was proportional to the range, 
rather than to the square of the range. Furthermore, 
the magnitude of the range error was found to be 
even smaller than extrapolation from the field data 
would suggest. This difference may be attributed to 
the more accurate controls which were possible in 
the laboratory study. 

The Harvard group then made an end-point 
analysis of stereoscopic acuity in the laboratory. The 
laboratory analysis of the properties of the stereo 
end-point was conducted in two series of experi- 
ments. The first series was carried out for free space, 
that is, the observations were made with the unaided 
eye. The second series involved observations under 
the conditions of telescopic vision. An a priori anal- 
ysis indicated that there are three primary visual 
cues available to the obsen^er for distance judgment 
when a target recedes from or advances towards the 
observer as in the magnification experiments. These 
cues are: (1) changes in the binocular parallax angle; 
(2) changes in apparent size; and (3) changes, at the 
observer’s eye, in the wave front of the light arriving 
from the moving target. 

Experiments were performed to determine the 
observer’s acuity (1) when all three changes were 
simultaneously available, (2) for each of the cues 
taken individually, and (3) for different pairs of 
cues. The results for free space showed that the 
greatest precision, consistency, and sensitivity were 
obtained when all three cues were simultaneously 
available for distance determinations. 

The relative contribution of each of the three 
visual cues was a variable factor, depending upon the 
range at which the results were obtained. At near 
distances, where the change in disparity which ac- 
companies a small change in range is large, disparity 
was found to be a very important cue; while at more 
remote distances of approximately 50 yards, where 
the binocular parallax angle is small, the disparity 
cue was relatively unimportant. At each range, how- 
ever, elimination of any single cue resulted in an 
increase in the error of the adjustments. The increase 
in error resulting from elimination of the size cue 
was very small at all ranges, indicating that size 
change was not so important for distance judgments 
as the wave front and the disparity cues. It was also 
found that the observer’s sensitivities to the three 


individual cues, when measured independently, 
summated to give the sensitivity which was obtained 
for the end point in which the three cues were 
simultaneously available. This summation of sensi- 
tivities seemed to suggest a reasonable basis for the 
high acuity found in the original field experiments, 
where all cues were available. 

A similar experimental analysis was then carried 
out for observations in telescopic space, using a num- 
ber of different magnifying powers from lx to 40x. 
The net result was the same as in the previous ex- 
periment in which observations were made with the 
unaided eye. The greatest acuity was found for the 
end point comprising all three cues. The elimination 
of any one cue always resulted in a loss in precision, 
consistency, and sensitivity. The loss resulting from 
elimination of the size cue was again found to be 
very small, indicating that the change in apparent 
size is a relatively unimportant factor. However, in 
contrast to the results for free space, in telescopic 
vision the changes in wave front per se are so re- 
duced by the telescope that this cue alone was in- 
adequate for the perception of changes in apparent 
distance. Nonetheless, when this cue was eliminated 
from the visual end point, the measurements suf- 
fered a considerable loss in precision, consistency, 
and sensitivity — indicating that changes in wave 
front, even when considerably reduced, do contrib- 
ute in an important way to the efficiency of the end 
point for distance determinations when combined 
with another cue. Of all three cues, the contribution 
provided by binocular retinal disparity was found 
to be of greatest relative importance. The results 
obtained when binocular parallax was the only avail- 
able cue, however, were poor as compared with the 
results for the end point comprising all three cues. 
This study is being continued under telescopic con- 
ditions using instruments designed to magnify rather 
than reduce the wave front differences. 

This group of experiments performed at Harvard 
University have demonstrated that stereoscopic acu- 
ity is a complex variable, whose magnitude depends 
on the cues available to the observer. When all the 
proper cues are available, stereo acuity is much 
higher than considerations based on the geometry 
of parallax per se would lead one to expect. This 
demonstration suggests the possibility of designing 
and constructing a portable short-base range finder 
for which the range error would be of the order of 
1 to 2 per cent. This development is under way at 
the Harvard Laboratory (March 1945). 


RESTRICTED 




Chapter 3 

FIELD COMPARISON OF INSTRUMENT TYPES 


3* INTRODUCTION 

A NUMBER of field studies were conducted at Fort 
. Monroe in which instruments of different types 
were operated simultaneously against fixed ground 
and aerial targets in order to obtain comparative 
data in regard to their performance. These compara- 
tive tests were all conducted by the Princeton Labora- 
tory at Fort Monroe. 

3 2 COINCIDENCE AND STEREOSCOPIC 
RANGE FINDERS 

The first of these reports is concerned with the 
comparative test of coincidence and stereoscopic 
range finders. (353) In these tests the American 
stereoscopic Height Finder Ml was operated against 
the British coincidence type Range Finders FQ 25 
and UB 7, in ranging on fixed ground targets, mov- 
ing naval targets and moving aerial targets. The 
coincidence and stereoscopic methods utilize the 
same basic principles of geometrical optics for the 
determination of the distance to a target. The two 
methods differ radically, however, in the nature of 
the criterion presented for human judgment. These 
British instruments were of the split field coincidence 
type. American crews were being trained at Fort 
Monroe to operate the coincidence instruments but 
this plan was dropped when six British seamen, who 
were experienced range takers, were made available 
for the tests. Until recently the British Services had 
tended strongly to the coincidence type of instru- 
ment while the American Services had adopted the 
stereoscopic principle for long-base instruments at 
least. The decisions of both the British and Ameri- 
can Services apparently grow out of different inter- 
pretations of the experience of the Battle of Jutland 
in World War I and are of no concern in this place. 

Tests were run in November and December 1941 
using the British seamen on the British instruments 
and experienced American observers on the Stand- 
ard Ml. Bad weather conditions and various experi- 
mental difficulties and mishaps made it impossible 
to obtain a really satisfactory quantity of data before 
the tests had to be terminated. Fixed target readings 
were made on targets from 2,700 to 14,500 yards. Only 


five aerial courses could be recorded and these were 
all level flight courses, at altitudes of 3,000 to 4,000 
yards and slant range between 4,000 and 12,000 yards. 
Continuous contact was used. Nine courses were 
obtained on slow moving naval targets at ranges from 
4,000 to 12,000 yards. In these latter courses continu- 
ous and broken contact were used at different times. 

It was found, throughout the tests, that the per- 
formances of the various instruments were more 
nearly alike when measured in external units (re- 
ciprocal range) than when measured in terms of 
error at the observer’s eye, in spite of marked differ- 
ences in physical dimensions of the instruments. The 
American Ml has a base length of 4.5 yards and used 
12 power; FQ 25 with a 6-yard base used 28 power and 
UB 7, a portable instrument, has 25 power and a 
3-yard base. The coincidence instruments did not use 
internal adjusters but were calibrated on targets of 
known range. In other words, the net performances 
of the different instruments were essentially com- 
parable although the instruments exhibited varying 
degrees of efficiency in performance relative to their 
size. On aerial courses precision errors of the four 
instruments were about alike when measured in re- 
ciprocal-range units. In UOE, the FQ 25 had com- 
paratively poor precision, while the UB 7, for three 
of the five aerial courses, had very small precision 
errors. The number of aerial courses was too small 
to yield much information about consistency of ob- 
servations from one course to the next. 

For the naval target courses, one American instru- 
ment was not operating. Precision errors of the other 
three instruments were similar to those on aerial 
height courses. In reciprocal-range units the three 
instruments had comparable precision. In UOE the 
FQ 25 was worse and the UB 7 was better than the 
American Ml. Consistency error of the UB 7 was 
smaller than that of the Ml, even when measured 
in reciprocal-range units, while the FQ 25 was similar 
in consistency to the Ml, again in reciprocal-range 
units. 

On ground targets the same general situation 
holds. Consistency errors of the four instruments over 
the 9-day period were the same when measured in 
reciprocal-range units. Again the UB 7 was better 
than the stereoscopic instruments in UOE and the 


17 



18 


FIELD COMPARISON OF INSTRUMENT TYPES 


FQ 25 was worse. Consistency over the 9 days was not 
perceptibly worse than daily consistency for any of 
the instruments. In other words, the readings over 
the 9 days did not scatter in total more than did 
readings for a typical day. 

An analysis of these results leads to the following 
conclusions. (1) Performance of the coincidence and 
stereoscopic instruments was about the same when 
range errors were measured in yards. (2) The UB 7, 
however, with a virtual base length smaller than that 
of the American stereoscopic instruments, was more 
efficient than the stereoscopic height finders in terms 
of performance for its size, while the large coinci- 
dence instrument, the FQ 25, was less efficient in this 
sense. This situation held for all types of target — 
fixed ground, naval, and aerial. (3) The UB 7 is some- 
what better than the American instrument in con- 
sistency on naval targets, even when measured in 
external units. 

This report is attached, as supporting data, to a 
Report to the Services issued by the Fire Control 
Division of NDRC. (20) This points out that the 
tests indicate no important difference in the preci- 
sion obtainable from the two types of instrument- 
coincidence and stereoscopic. They do indicate, how- 
ever, that the difference in performance between 
large and small instruments is by no means as great 
as would be anticipated from simple geometrical 
optics. The report concludes with the belief that 
stereoscopic and coincidence acuities are about 
equal. Under favorable conditions existing instru- 
ments of the two types perform about equally well, 
and the choice between them for any given purpose 
must be based on matters of convenience related to 
the particular conditions under which they are to 
be used. 

Ortho-pseudo Modification 

A second comparative study was made at the 
Princeton Laboratory at Fort Monroe between the 
standard Ml Height Finder and an instrument modi- 
fied to the ortho-pseudo system developed at East- 
man Kodak Company. (351) This instrument is de- 
scribed in an Eastman Kodak report (204) and will 
be described in some detail in Section 17.3, Chapter 
17, Part IV. Here it is sufficient merely to indicate 
that the Eastman design is of the stereoscopic sort 


and was built into the case of the standard M 1 Height 
Finder. Instead of a fixed reticle it presents to the 
observer a field in which he sees two images of the 
target. Manipulation of the range knob causes one 
of these images to apparently approach and the other 
to apparently recede from the observer. The task of 
the observer is to bring the two images of the target 
to the same stereoscopic distance on each side of a 
dividing line through the horizontal diameter of the 
field. This instrument was shipped on December 22, 
1941 from the factory at Rochester, New York. 

Comparative tests were made from January 
through April 1942 after experienced stereoscopic 
observers were given a week’s practice with the ortho- 
pseudo modification. All instruments were charged 
with helium for temperature stability. During the 
first month a systematic schedule of tests on aerial 
courses was followed, using courses explicitly selected 
for this purpose. Level and dive courses, and cross- 
ing, approaching and receding courses at various 
ranges were included. All of these courses were run 
at an altitude of 10,000 feet. Observations on fixed 
ground targets were also included during this period. 
Early in March other observations were taken on 
aerial courses at altitudes of about 7,500 feet and of 
the level crossing type. Later in March and through 
April, observations were taken on ground and aerial 
targets using, as observers, students in the Stereo- 
scopic Observers’ Course of the Coast Artillery 
School. 

It is not easy to estimate the performance limits 
theoretically attainable by the Ml Height Finder. 
There is some basis for expecting that the instrument 
ought to have, in reading height of an aerial target 
at, say, 2,000 yards altitude and 5,000 yards slant 
range, a precision error of about 5 yards and a con- 
sistency error of about 1 or 2 yards. Actually, preci- 
sion and consistency of this order are not often at- 
tained. A realistic estimate of performance at that 
time, under good conditions, was a precision error 
of about 10 yards and a consistency error of 12 or 
more yards for the aerial target described above. 
There was good reason to expect that the ortho- 
pseudo instrument should have a smaller precision 
error than the Ml Height Finder. This is true be- 
cause of the fundamental difference in the methods 
used in the two instruments to measure distances. 
To the observer, the apparent depth separation be- 
tween the ortho and pseudo images is twice the sep- 


RESTRICIED 


ORTHO-PSEUDO MODIFICATION 


19 


aration between target image and reticles in the M 1 
instrument. Doubling the depth separation theoreti- 
cally should improve precision by a factor of two, 
since the apparent error in settings is twice as notice- 
able to the observer as it would be for the same set- 
ting on the standard instrument. Also the absence of 
reticles in the ortho-pseudo instrument should tend 
to eliminate the difficulties associated directly with 
the reticle in the Ml Height Finder. Haze, for ex- 
ample, between the observer and the target creates 
a distance cue which disturbs the comparison with a 
reticle which is not affected by haze. Since the com- 
parison in the ortho-pseudo is between essentially 
identical images, the presence of haze should cause 
less difficulty. Another advantage derived from 
leaving out the reticles lies in the fact that the azi- 
muth position of the target is then unimportant to 
the observer as long as the target is in the field of 
view. In the reticle instrument, this centering of tar- 
get near the fiducial line of the reticle makes a differ- 
ence to the observer, as is indicated in Chapter 10 
of this report. On the other hand, poor elevation 
tracking, which causes the target image to move up 
and down in the field of view, might cause consider- 
ably more difficulty in the ortho-pseudo arrangement 
than in the Ml instrument, because of the split field 
in the former which causes the two images to move 
in elevation in opposite directions. 

On the basis of approximately 5,000 recorded 
observations made by 12 observers on the ortho- 
pseudo instrument the following results were found. 
Throughout the test, the performance on the ortho- 
pseudo was always at least as good as that of the 
standard instrument. On level aerial courses, the 
ortho-pseudo sometimes proved superior to the 
standard instrument, in either precision or con- 
sistency or both. The extent and character of the 
superiority varied from observer to observer. Two 
highly experienced observers showed consistency 
about twice as good on the ortho-pseudo as on the 
standard Ml; the consistency of an equally experi- 
enced observer was about the same for both instru- 
ments. The situation was similar with respect to 
precision, which was about twice as good on the 
ortho-pseudo for two observers and the same for one. 
Hence, for one observer the precision and consistency 
were both better for the ortho-pseudo, and for the 
other two observers, one had better consistency and 
the other better precision with this instrument. 


There was no detectable difference in performance 
of the two instruments on dive courses. The dive 
course performance of both instruments was so poor 
that differences of the order of magnitude found 
elsewhere could not have been detected. 

On fixed ground targets the two instruments per- 
formed about equally well except in the case of 
student observers on the 14,000 yard target. On this 
target, both their consistency and precision were 
better for the ortho-pseudo than for the Ml Height 
Finder by a factor of about two. The performance, 
with the ortho-pseudo of four experienced stereo- 
scopic observers on all fixed targets and of the stu- 
dent observers on the 2,920 yard target was about the 
same as that on the standard instrument. Since the 
Ml Height Finder was a more familiar instrument to 
the observers than the ortho-pseudo, it is possible 
that the relative merit of this latter instrument might 
be somewhat underestimated. However, as tests were 
run which determined no real learning with the 
ortho-pseudo instrument by already experienced 
stereoscopic observers, this factor is probably not 
important. No real learning seemed to take place 
since all men read as well within a few days as they 
did at the end of the several months test. It is inter- 
esting to note that whenever the instruments were 
different in performance, the ortho-pseudo was the 
better by a factor of about 2. Although this factor of 
2 is in agreement with theoretical expectation, this 
does not prove that the doubling of the angular 
magnification in the ortho-pseudo was entirely re- 
sponsible. 

The performance of all observers with the ortho- 
pseudo on level aerial courses did not show improve- 
ment in both precision and consistency over perform- 
ance on the standard instrument. Each observer 
showed improvement in at least one or the other. 
One observer was more consistent with the ortho- 
pseudo, another read with greater precision, and the 
third showed improvement in both types of error. 
The results from these three most experienced ob- 
servers, therefore, indicate that it cannot be asserted 
that, regardless of the observer, the ortho-pseudo is 
superior to the standard Ml with respect to either 
consistency or precision. What can be said is that 
the ortho-pseudo is at least as good as the Ml with 
regard to either type of error, and that each of the 
test observers showed improvement in at least one 
type of error in reading on the ortho-pseudo. 


RESTRICTED^ 


20 


FIELD COMPARISON OF INSTRUMENT TYPES 


In regard to course corrections, two observers gen- 
erally read shorter on the ortho-pseudo than on the 
standard instrument and the third test observer read 
longer. For the two observers who read short, there 
was also a tendency for the corrections to have less 
scatter on the ortho-pseudo than on the standard 
instrument. 

On the basis of these findings, the Princeton Lab- 
oratory recommended, other things being nearly 
equal, that the ortho-pseudo type of instrument be 
preferred over the standard reticle-type instrument. 

These data are attached to a Report to the Services 
issued by Fire Control Division of the National De- 
fense Research Committee. (21) After summarizing 
the situation this report states that, when all the 
available information is taken into account, there 
appears to be good reason to believe that a well- 
designed ortho-pseudoscopic range finder in the 
hands of a well-trained crew would be more accurate, 
and more consistently accurate, than any other type 
of instrument so far devised. In time of peace, or if 
the procurement of optical range finders were not 
so difficult, an energetic prosecution of this develop- 
ment would obviously be warranted. Under the then 
existing circumstances, the situation was not quite 
so clear and the opinion of the Services as to the 
desirability of further work at that time was invited. 

Detailed results of the experiments performed dur- 
ing the first month of the comparative ortho-pseudo 
Ml tests will be found in a study by the Fort Monroe 
Princeton Laboratory. (511) These results have to 
do with observations on fixed targets by experienced 
and student observers and with aerial courses of both 
level and dive type. 

3.2.2 xhe German R 40 Range Finder 

The Aberdeen Proving Ground reports compara- 
tive tests of the German 4-meter range finder R40 
with the standard U. S. Height Finder Ml. (46) The 
experiment was to determine the relative accuracy of 
the two instruments. The German instrument is 
described in a Frankford Arsenal report. (228) The 
instruments were tested by reading slant ranges to 
three kinds of targets: fixed terrestrial targets, the 
moon, and aerial targets. The range readings were 
converted into convergence angles and compared for 
both precision and consistency. The internal adjust- 
ments were made on fixed targets at known ranges. 


It was found that the precisions with which it was 
possible to read on fixed targets were 0.492 seconds 
of arc with the Ml and 0.624 seconds with the R40. 
On the moon the precisions were 2.32 seconds for 
the Ml and 2.27 seconds for the R40 and the accura- 
cies were 3.27 seconds for the Ml and 5.76 seconds for 
the R40. On the airplane targets, the positions of 
which were determined by ballistic cameras, the ob- 
servers read slant range by the continuous contact 
method and photographs of the range dials were 
taken once a second. The precisions for aerial targets 
were 2.76 seconds for the Ml and 3.38 seconds for the 
R40 and the accuracies were 3.74 seconds and 4.74 
seconds respectively. The Ml was charged with 
helium and the R40 was not so charged. 

On the basis of these results, the Aberdeen group 
concludes that the apparent superiority of the Ml 
instrument is real but that the difference between the 
two instruments is small. They further state, in all 
tests with both instruments, any errors due to tem- 
perature stratification of the gas inside the instru- 
ments were small enough to be completely covered 
up by the other errors present. These results cannot 
be considered as final, because an examination of 
the tabular data indicates that there were large ob- 
server differences. Furthermore, the observers were 
not rotated between the instruments. The question 
can therefore be raised as to whether the differences 
in performance are due to the fact that the Ml ob- 
servers were on the average slightly better than the 
R40 observers, rather than to any instrumental differ- 
ences. Another unfortunate aspect of this study is 
that no comparison was made between the two in- 
struments in regard to consistency or day-to-day 
variation of range readings. It is especially in respect 
to greater long-time stability that one might expect 
the German instrument to be superior to the Ml 
because its construction is aimed to this end. 

At the Aberdeen Proving Ground, comparative 
tests were made of the German R40 range finder and 
the American Height Finder Ml, both reading slant 
range. (47) Readings were taken on fixed terrestrial 
targets, the moon, and aerial targets. Internal adjust- 
ments were made on fixed targets at known range. 
The superiority of the Ml instrument was demon- 
strated throughout for better consistencies of internal 
adjustment readings and better accuracies and con- 
sistencies for all three sorts of external target. The 
differences between the two instruments are small, 
however. 


RESTRi 


Ml HEIGHT FINDER VS MICKEY RADAR 


21 


3.2.3 Mickey 

Still another comparative study by the Princeton 
Laboratory at Fort Monroe reports the performance 
of the Ml stereoscopic Height Finder in ranging on 
aerial targets with “Mickey,” an early form of radar 
developed for antiaircraft ranging. (361) Mickey 
was later adopted by the Army as SCR-547. This test 
was made in August 1941, early in the development 
by the Bell Telephone Laboratories of portable 
radar sets to be used for this purpose. In all, 125 aerial 
courses were flown and about 50 of these were ana- 
lyzed. The range varied from 1,000 to 18,000 yards. 
In some courses the range was constant, in others it 
increased or decreased at varying rates, or reversed 
its direction several times. Both level and dive courses 
were included. Highly experienced stereoscopic op- 
erators were used on the experiment. The statistical 
analysis of the results indicates, for Mickey, the pre- 
cision error ranged from 5 to 78 yards, with a mean 
precision error for all courses analyzed of 27 yards. 


Seventy-five per cent of all precision errors lay be- 
tween 16 and 36 yards. The consistency error for 
Mickey is estimated at 25 yards. Average course errors 
ranged from —16 yards to 87 yards with a mean of 
25 yards and standard deviation of 25 yards. Sixty- 
eight per cent of the average course errors lay be- 
tween — 1 and 45 yards. The standard deviation of 
total range error is about 25 yards up to 7,000 yards 
range. It then increases, averaging 33 yards for all 
ranges up to 10,000 yards. The range error is rela- 
tively independent of range for Mickey as compared 
with the Ml Height Finder, which is subject to errors 
proportional to the square of the range. The optical 
Height Finder Ml appears superior to the radio 
range finder Mickey for ranges less than 3,000 yards; 
they are equal for ranges between 3,000 to 5,000 yards; 
and Mickey appears superior for ranges greater than 
5,000 yards. Experiments designed to discover some 
of the major sources of error in Mickey were un- 
successful. 


RESTRICTED 





PART 1 


ERRORS IN STEREOSCOPIC INSTRUMENTS 
AND THEIR CONTROL 


E rrors in ranging may be classified as: (1) those 
inherent in the design, construction and adjust- 
ment itself; (2) those brought about by external 
physical conditions such as disturbances in the at- 
mosphere through which the target is viewed, and 
(3) those errors which are inherent in the operator 
or his method of operating the instrument. Chapters 
4 to 8 deal with the first of these categories. The 
other two are dealt with in Chapters 9 to 12. 

Sources of error in the instrument itself had not 
been analyzed at the beginning of this war, although 
such inherent errors were known to exist. The studies 
reported in these chapters show how, after the con- 
tributing elements were located, each factor was in- 
vestigated independently of the others and a cor- 
rective devised for each defect. 

Failure to focus the image uniformly in the reticle 
plane is called perspective error. Sections 4.1 and 4.2 
present a group of studies which show that if the 
light beam through the objective lens can be re- 
stricted to the central portion of this lens, these 
errors will usually be reduced. This narrowing of 
the beam, and its limitation to the central portion of 
the optical system, can be accomplished by cutting 
down the size of the circular openings at the end 
windows or by introducing similar stops near the 
objectives. In the design of new instruments, adjust- 
able internal stops are recommended, but perspective 
errors in existing range finders can be reduced by 
the addition of adjustable end-window stops. In 
either case, perspective errors are least when the 
openings are as small as are consistent with getting 
a sufficient amount of light through the optical 
system. 

The improper spacing of the eyepieces, or im- 
proper interocular setting, usually causes ranges read 
too short when the eyepieces are too near together 
and too long when the eyepieces are too far apart. 
Section 4.3 gives a digest of a group of studies which 
relate the interocular setting to the interpupillary 
distance [IPD] or to the interaxial distance [lAD] of 
an observer’s eyes. The IPD is the distance between 


the centers of the pupils of the eyes and the lAD is 
the distance between the axes of vision of the eyes. 
These two distances are not identical in general. The 
experiments indicate that the interocular setting 
should be based on the IPD and methods for measur- 
ing this accurately have been devised. 

Temperature errors may arise (1) when the whole 
instrument is changed uniformly from one tempera- 
ture to another, (2) where temperatures are constant 
with time but variable from one part of the instru- 
ment to another, and (3) where changes in tempera- 
ture are taking effect. Effects of the first category, 
such as changes in lens curvature and lengthening of 
the optical bar because of uniform temperature vari- 
ation can be made self-compensating to a consider- 
able extent by careful design. 

Chapter 5 deals chiefly with effects of the second 
and third categories. When the sun shines on the top 
of a range finder, a temperature stratification takes 
place in the gas inside the tube, causing a variation 
in the refractive index of the gas from top to bottom 
of the instrument. The British have alleviated this 
effect by stirring the air within the tube. The Ameri- 
can solution has been to charge the tubes with helium. 
Such a solution has created new problems of refocus- 
sing and the like as well as problems of charging the 
tube with helium and of maintaining a high per- 
centage of helium within the instrument. Sections 
5.1, 5.2, and 5.3 of Chapter 5 outline the experiments 
which have led to the solution of the temperature 
problems as regards stratification of the internal gases 
in existing instruments. The remainder of Chapter 5 
deals with experiments which have led to the de- 
velopment of range finder parts more thermally 
stable than those in existing instruments. 

Chapter 6 discusses the problems of power and 
base length of ranging instruments. It has been known 
that increased base length and increased magnifica- 
tion both fail to give the theoretically expected in- 
crease in acuity. Certain present experiments confirm 
these earlier findings in regard to both variables. 
This experimental work on this problem is by no 


23 



24 


ERRORS IN STEREOSCOPIC INSTRUMENTS 


means completed and, indeed, our knowledge is not 
far enough advanced to warrant specific recom- 
mendations. 

Chapter 7 deals with experiments on calibration 
of range finders in the field — the expected accuracy, 
methods of calibration, particularly emphasizing the 


use of celestial targets and a study of reticle design 
for the internal adjuster target. Chapter 8 of the 
report deals with a small number of miscellaneous 
instrument and operational defects such as penta- 
prism rotation, use of filters, leveling and alignment 
errors and the like. 


(restKICTed^ 


Chapter 4 

PERSPECTIVE ERROR 


41 INTRODUCTION 

P erspective error is an error due to the fact that 
the images formed by the objectives do not lie 
exactly in the planes of the reticles but are removed 
from the reticle planes by an equal amount for both 
the right and left sides. Such a condition can be due 
to any or all of three causes: (1) improper adjust- 
ment of the instrument; (2) the use of the instrument 
at a range other than that used for adjustment; or 
(3) thermal changes in the instrument which result 
in expansion of the optical bar, change of focal 
length of the objectives or curvature of the end re- 
flectors. It leads to a difference in observed range 
when different interocular settings are used. 

The origin of these effects is the fact that light 
coming from a target through different portions of 
an objective intersects the reticle plane at slightly 
different points. This is expected theoretically, and 
has been demonstrated experimentally by placing 
stops over the objectives or end-windows in such a 
way as to expose only a small portion of the window, 
and observing that the range reading varied with the 
portion exposed. With the window wide open, the 
images formed by the different portions are super- 
imposed, and thus a certain degree of blurredness 
results. With the instrument stopped down, the 
image is sharper and the range readings correspond- 
ingly better. Furthermore, when the instrument is 
stopped down, the observer uses the area of the op- 
tical elements through which the internal adjuster 
beams pass, and hence the RCS adjustment can be 
expected to be more reliable than with end-windows 
unstopped. 

4 2 CONTROL BY END -WINDOW STOPS 

Early experiments at the Princeton Laboratory at 
Fort Monroe indicated that the placing of opaque 
stops over the end-windows of Ml and M2 greatly 
reduced these perspective error effects. This led, as 
early as February 1942, to a Report to the Services 
issued by the Fire Control Section of NDRC. (3) 
Diaphragms with a 1-inch aperture were placed be- 
fore the end-windows of the instruments. All studies 
were made on fixed targets of known range. 

Approximately 1,000 readings were taken in these 


early experiments. It was found that the average of 
an observer’s readings was closer to the true range 
with the windows stopped down than with them 
open. This was true for all observers. The improve- 
ment is not uniform for all observers, being 20 per 
cent in the case of the observer who showed least 
improvement, while in the case of the observer who 
showed greatest improvement, the errors with the 
instrument stopped down were only one-sixth as 
great as with it wide open. A modest reduction in the 
spread of an observer’s readings— a modest increase 
in the precision with which he makes his settings— 
was produced by stopping down the instrument. In 
this regard, one observer showed improvement of 
6 per cent, one 12 per cent, while the other three were 
substantially unchanged. Also, an observer’s readings 
varied less from hour to hour with the instrument 
stopped down than with it wide open. This was true 
of four out of the five observers, the improvement 
ranging from 19 to 39 per cent. One observer showed 
greater variability by approximately 20 per cent. 

The report ends with the following recommenda- 
tions to the Services: (1) That variable diaphragms, 
suitable for external attachment to the end-windows, 
be provided for all existing range and height finders. 

(2) That range finders now in course of construction 
be equipped with suitable variable diaphragms. 

(3) That all observers be instructed to use their in- 
struments stopped down as much as conditions of 
target illumination will permit. This will be at all 
hours except a very few minutes at dawn and dusk, 
and perhaps occasional days of unusually low visi- 
bility. Under such conditions of poor illumination 
the instrument must be used full field to gain suffi- 
cient light gathering power. 

A report from the National Bureau of Standards 
discusses these results from a theoretical point of 
view and points out that perspective errors may be 
due to a number of other factors. (321) 

The Princeton Laboratory at Fort Monroe has 
given an elementary discussion of perspective error 
in range finders. (365) This general theory forms the 
basis of all the experimental work on this topic by 
the Princeton Laboratory. 

Further experiments with end-window stops of 
1 -inch aperture are reported by the Princeton Labora- 


ESTRTCTFD 


25 


26 


PERSPECTIVE ERROR 


tory at Fort Monroe. (350) The results indicate that 
the use of a small central part of the height finder 
optical system for ranging does improve certain char- 
acteristics of height hnder performance. These ex- 
periments are based upon the following theoretical 
considerations. If there is a suitable kind of differ- 
ential temperature stratification in a height finder or 
lack of planarity of end reflectors, then rays passing 
through different parts of the end-window will be 
focused on slightly different places in the reticle 
plane. This may have three effects: (1) a diffusion 
of the image; (2) a shift of the center of gravity of 
the image, or (3) a shift of the center of gravity of 
the whole image relative to the center of gravity of 
that part of the image using the same part of the end 
reflector as the internal adjuster image. The effect 
of the first would be expected to lead to irregular 
performance, of the second to lead to errors cor- 
rected by the internal adjuster, and of the third to 
both long- and short-term errors uncompensated for 
by the internal adjuster. 

The first experiment studied the magnitude of 
such errors when a 1-inch aperture was placed in 
front of the left end-window of a height finder and 
eccentric stops were placed in sequence in each one of 
four positions above and below and to each side of 
the center of the right end-window. The centers of 
these four eccentric stops were, in each case, ^ inch 
from the center of the end-window. The results, for 
a fixed target at 2,915 yards show variations, for the 
four positions from 2,869 to 2,926 for one observer 
and 2,901 to 2,958 for another. Results with concen- 
tric l-inch apertures on both end-windows for fixed 
targets indicate that: (1) the change in net correction 
between 2,915 and 4,015 yards was reduced by 30 per 
cent; (2) the difference between observer’s daily 
means was greatly reduced; (3) the spread within 
sets of five range readings was reduced about 5 per 
cent, which is nearly statistically significant for the 
5,500 readings involved; and (4) the variability of 
observers’ daily means was not significantly changed. 

A second report from the Princeton Laboratory at 
Fort Monroe deals with reduced apertures employing 
aerial targets. (359) Full field and 1-inch apertures 
were employed using 12x and 24x power in both 
cases. Six standard Ml Height Finders and 17 better 
and 18 poorer observers were used. The results show 
that consistency at reduced aperture was found to be 
much better than at full aperture, using either 24 or 
12 power. Precision with 24 power was not twice as 


good as that with 12 power, which ratio is theoreti- 
cally expected when the observer’s sensitivity is the 
controlling factor. These findings for aerial targets 
agree with those already reported for ranges taken 
on fixed ground targets using reduced power or re- 
duced aperture separately. The improvement in con- 
sistency is related directly to the size of the exit pupil, 
the smaller the exit pupil, the better the perform- 
ance. Readings of various observers on the same in- 
strument tend to be more nearly the same when exit 
pupil size is decreased by reduced aperture or in- 
creased magnification. During these tests, no calibra- 
tion adjustments were made on any of the instru- 
ments, since the primary purpose was a study of the 
observer’s precision rather than their absolute accu- 
racy. Internal adjuster readings were also eliminated, 
thus permitting nearly uninterrupted reading of 
height. All instruments were charged with helium. 
The weather was clear through the 2-day test. The 
targets flew crossing courses at about 10,000 feet alti- 
tude, with slant range starting at about 14,000 yards, 
decreasing to about 8,000 yards and then increasing 
to about 14,000 yards. With 24 power the precision 
errors were 2.7 UOE for both 1-inch and full aper- 
ture. For 12 power the precision errors for full aper- 
ture were 4.1 UOE and this was reduced to 3.2 UOE 
when the reduced aperture was used. Consistency 
errors were greater for 12 than for 24 power and 
greater for full aperture than for reduced aperture 
with either power. 

Some observer reactions are given in this report 
which are of interest. The observers reported diffi- 
culty in using a height finder whose eye distance was 
greatly reduced. Reduction in eye distance takes 
place when, as in these present tests, the aperture is 
reduced at the end-window. If the aperture were 
reduced at or near the objective, the eye distance 
would be unaffected and the reduction of the aper- 
ture should not produce physical discomfort and 
fatigue. During some informal observations made 
during a short test at the Naval Gun Factory, the 
observers reported that they perceived a much 
greater change in apparent depth for a given change 
in range with the instruments stopped down. This 
may be explained if, improved by depth of focus, 
the reduced aperture reduces the blurring of the 
image. When one clear and one blurred image are 
presented with binocular disparity, there is an un- 
conscious tendency to rejudge the blurred image and 
bring it nearer to the sharp one. Thus, if the target’s 


CONTROL BY END-WINDOW STOPS 


27 


image is blurred at full aperture, the reticle should 
attract the target toward its own depth. Finally, the 
reduction in aperture reduces the apparent illumi- 
nation of the image and this fact was noticed by the 
observers. Each man was asked if the reduction in 
intensity made it more difficult to use the height 
finder and they all said that it did not. 

Another report from the Princeton Laboratory at 
Fort Monroe presents further experimental data to 
support the recommendation that stereoscopic range 
finders be used at reduced aperture. (364) The report 
indicates the range errors may occur as a result of 
instrumental focus errors when the observer’s inter- 
ocular setting or eye position is incorrect. Following 
a theoretical discussion which shows the perspective 
effect produced if the target and the images of the 
reticles are not all in the same plane so that shifting 
a point of view disaligns the target and correspond- 
ing reticle image, the experimental verification is 
given. Results indicate that the errors intentionally 
introduced by incorrect interocular settings are much 
reduced when the 1-inch aperture is placed before 
the end-windows and still further reduced when the 
aperture is placed at the objective, when either 12 or 
24 power is used. However, the dependence of range 
on interocular setting varies with the observer and 
the slope found using a 1-inch stop is of the same 
order of magnitude as the difference between ob- 
servers. 

These studies are gathered together in a second 
Report to the Services issued by the Fire Control 
Section of NDRC. (18) It is pointed out that in an 
ideal stereoscopic range finder the target images 
formed by the right- and left-hand objectives would 
lie accurately in the planes of the right- and left- 
hand reticles. In an actual instrument they are not 
there for various reasons, the principal ones being: 
(1) imperfections in adjustment of the objective and 
reticle during assembly; (2) temperature changes in 
the optical bar, which alter the distance of the reticle 
from the objective, and in the objective itself which 
alters its focal length; (3) the fact that, even if the 
instrument were perfectly adjusted for one target 
distance, the images of targets at other distances 
would not be on the reticle plane. 

It has been found by these studies reported that 
no errors result if the interocular distance is cor- 
rectly set and the two sides of the instrument are 
accurately matched, i.e., if the distances of the target 
images from their reticle planes are equal in both 


size and sense. If the two sides of the instrument are 
not matched, lateral head movements will cause 
range errors. Whether or not the two sides of the 
instrument are matched, range errors will occur un- 
less the correct interocular distance is set into the 
instrument. Under controlled conditions, errors as 
large as 30 UOE have been observed in using the 
Army Ml Height Finder when an improper inter- 
ocular setting was put into the instrument. It is 
believed on both theoretical and experimental 
grounds that perspective errors of 5 to 20 UOE may 
be common under service conditions with virtually 
all types of precision stereoscopic range finders. Such 
errors can be materially alleviated by reducing the 
size of the exit pupil. This is most conveniently done 
by means of diaphragms located near the objectives 
or the end-windows. The better location is near 
the objectives. 

Recommendation of Variable 
Diaphragms 

A previous Report to the Services had already 
recommended that all range finders be provided with 
variable diaphragms and that observers be instructed 
to use the instruments stopped down as much as the 
conditions of external illumination will permit. (3) 
In the second report, this recommendation is again 
reported with the additional observation that, in 
new instruments the diaphragm should be located 
near the objective. Diaphragms placed at the end- 
windows are not quite so satisfactory, but because 
of ease of attachment may be preferable in the case 
of instruments already in the field. Fixed stops are 
not recommended because the full light-gathering 
power of the instrument may be required in some 
important tactical situations. 

In another previous Report to the Services, atten- 
tion was drawn, on other grounds, to the importance 
of correct interocular settings. (9) This is again em- 
phasized. It is now recommended that an accuracy 
requirement of 0.25 mm be placed upon the inter- 
ocular adjustment mechanism, and that it be re- 
quired to be self-locking. It is also recommended that 
care be exercised in keeping the objectives in proper 
focal adjustment. In particular, in helium-filled in- 
struments it is desirable to readjust the objectives 
when helium is adopted, and thereafter to maintain 
the helium content at such a level that the adjust- 


I KI-.STRTCTEi;^ 


28 


PERSPECTIVE ERROR 


merit is not impaired. The Report ends with a section 
on the theory of perspective errors. 

^•2-2 Detailed Studies of End -Window Stops 

Detailed reports of these studies will be found in 
several Fort Monroe Princeton Laboratory studies. 
In the first of these reports the findings were as fol- 
lows. (512) Definite improvement due to the use of 
end-window stops was found because: (1) variability 
of the median of five range readings is reduced, the 
probable error being cut about 30 per cent; (2) ob- 
server differences are both smaller and less variable; 
(3) observer net correction is reduced for each of five 
observers; and (4) the mean Curve B of several stu- 
dent observers was improved by 30 per cent. Prob- 
able improvement was found for differences in 
observers’ means for the same day which were prob- 
ably reduced by the use of stops. No significant im- 
provement was discovered when stops were used for 
variability of daily means, and variability within sets 
of five readings was changed probably less than 10 
per cent. 

Another Princeton Laboratory report is con- 
cerned with the interaction between dependence of 
range on eccentric stops and internal adjuster read- 
ings. (514) It was found that the range read by a 
height finder stopped down to 1-inch aperture de- 
pends on the location of the aperture over the end- 
window. It was established that (1) the effect due to 
horizontal displacement depends on the internal ad- 
juster reading and (2) the position of the RCS 
wedges is not responsible for this effect, either in 
helium or in nitrogen. The cause of this effect is 
probably thermal in origin. 

Another Princeton Laboratory report discusses 
the locations of stops in the Ml Height Finder. (516) 
It considers the effect of stops which are at or before 
the objective and gives a theoretical discussion of the 
possible positions at which such stops might be 
placed in the Ml Height Finder system and of the 
location and diameter of the stop images. 

The point of view from two circular stops is dis- 
cussed theoretically in another Princeton Laboratory 
report. (519) The report shows that the point of view 
of an optical system limited by two circular stops is, 
laterally, very nearly halfway between the sides of 
the useful bolt of light. 

Finally a Princeton Laboratory study discusses per- 


spective error in ortho-pseudo-stereoscopic range 
finders of the Mihalyi type. (526) This is again a 
theoretical study. The analysis shows that, with in- 
correct interocular setting or head shift in such an 
instrument, no perspective error may be expected 
with change of range to target, or symmetric de- 
focusing between target and beam splitters, includ- 
ing symmetric defocusing of objectives or defocusing 
between joiners and the eyes. No effect will be ap- 
parent for incorrect ocular setting when there is 
a symmetric defocusing between target and splitters 
but, with head movement, double the effect will be 
expected over that found in an ordinary stereoscopic 
range finder. A perspective error may be expected 
for both incorrect interocular setting and shift of 
head when defocusing exists between splitters and 
joiners. 

A report from the Frankford Arsenal, Princeton 
Branch, deals with designs of internal stops for 
height finders. (235) It is proposed to equip the Ml 
Height Finders, at the objectives, with externally 
operated, removable diaphragms. It is pointed out 
that a continuously adjustable iris diaphragm is not 
necessary; a simple “in and out” diaphragm with 
single-size stop would be satisfactory. The problem 
falls into two parts which may or may not be satis- 
fied by a single solution: (1) the design and construc- 
tion of objective stops to be installed in height finders 
during production, and (2) the design and construc- 
tion of objective stops to be installed in height finders 
already built and perhaps in the field. A continu- 
ously adjustable iris diaphragm stop devised by the 
Eastman Kodak Company is considered satisfactory 
but perhaps needlessly elaborate for the solution of 
the first problem. The report suggests a simple type 
of diaphragm and outlines steps for further de- 
velopment. 

Many of these matters are taken account of in a 
modification of a special Ml Height Finder for ex- 
perimental purposes reported by the Frankford 
Arsenal, Princeton Branch. (237) In order to reduce 
or study perspective error, the following modifica- 
tions were to be introduced: 

1. Focusable objectives so arranged that their dis- 
placement in and out of the optical bar is controlled 
from outside the instrument with a satisfactory de- 
gree of precision and the amount of shift indicated 
on some form of scale. 

2. Special reticles, modified by cementing small 
plane-parallel pieces of glass over the end reticle 


RESIRICTE 




IMPORTANCE OF CORRECT INTEROCULAR SETTING 


29 


marks in such a way that the focus of the images of 
these reticle marks, as seen at the ocular, will differ 
by approximately 1/2 diopter from the focus of the 
other reticle marks. 

3. A modified interpupillary scale on which read- 
ings of the position of the interpupillary adjustment 
could be made down to 1 /20 of a millimeter. 

4. A new interpupillary adjustment sufficiently 
precise and free from backlash to obtain satisfactorily 
a tolerance of 1/20 mm. 

5. Diopter scales graduated at intervals of 1/10 
diopter. 

6. Installation of the latest type of controlled 
apertures. 

7. Fitting of the height-adjustment knob with a 
graduated scale. 

Unified field modifications of the Ml Height 
Finder are proposed in a Princeton Branch report 
of the Frankford Arsenal. (247) These include an eye 
positioner by improvement of the present eye shields. 
The use of diaphragms had already been authorized 
by the Army. 

It will be noted that the use of diaphragms is 
merely a measure for reducing the effect of perspec- 
tive error in range finders. It by no means eliminates 
the causes of this error. Attempts at eliminating or 
reducing the causes of this error will be discussed in 
subsequent sections of this summary of experimental 
and theoretical work on range finders. 

4 3 the question OF THE 
INTEROCULAR SETTING 

Quite early in the NDRC study of stereoscopic 
range finders, it became evident that setting the ex- 
actly correct interocular measure between the two 
eyepieces was a critical matter. In the Tufts College 
Laboratory of Sensory Psychology and Physiology 
variations in the interpupillary setting of a Navy 
Mark 2 Stereo Trainer were made for three observ- 
ers, with the following extremes for the different 
subjects: 62-68 mm; 65-69 mm; and 61-64 mm. (556) 
In all three cases it was found that a linear relation 
existed between the magnitude of the constant error 
and the interpupillary distance set into the instru- 
ment. For the extreme settings, the differences of the 
constant errors for the three observers were respec- 
tively 8, 6, and 4 UOE. The constant errors were 
invariably large and negative (i.e. ranges read too 
short) when the interocular distance was too narrow. 


and smaller and positive (i.e. ranges read too long) 
when the interpupillary setting was too wide. 

Independently, similar observations were made at 
the Harvard Fatigue Laboratory, employing six ob- 
servers on the Harvard Fatigue Laboratory stereo 
acuity instrument. (265) Interocular distance was 
varied in both directions, approximately 5 mm on 
either side of the normal interpupillary distance, to 
the point where the observer could no longer fuse 
the reticles. The investigators were interested in 
variability of judgment rather than accuracy of 
ranging. The differences in interpupillary distance 
gave no constant trend in variability of observation 
for two of the six observers. For the other four ob- 
servers, the results indicate that there is an optimum 
value of interocular setting which gives the smallest 
amount of variability. When this optimum setting 
is violated in either direction, there is a resultant 
increase in variability which becomes progressively 
greater as the amount of violation is increased. For 
two observers, this increase in precision was con- 
siderably more marked for settings of the instrument 
narrower than the measured interocular distance; in 
one case giving a decreased precision of approxi- 
mately 20 seconds of arc, and for the other approxi- 
mately 23 seconds of arc, for the extreme narrow 
settings. 

Effect of Interpupillary Adjustment 

It was noted in this study that the optimal pre- 
cision did not occur exactly at the measured inter- 
pupillary distance for any one of the four observers. 
There is apparently a preferred setting which was 
narrower than the measured setting for one observer, 
and wider for the other three observers. These differ- 
ences between the measured interpupillary distance 
and the preferred interpupillary distance were rela- 
tively small— being 1 mm in three cases and 2 mm for 
the fourth observer. Such distances were substantially 
found to be of considerable importance. The in- 
vestigators believe that they may be due either to the 
fact that the best interocular distance differs from the 
interpupillary distance, or to errors in measuring the 
interpupillary distance. However, existing methods 
of measuring interpupillary distance are subject to 
errors of as much as 1 or 2 mm. All such methods 
assume that the lines of sight are maintained in a 
parallel position with fixation at infinity. Movements 


[r^tricted ^ 


30 


PERSPECTIVE ERROR 


of convergence from this position are normal and 
usual while movements of divergence are infrequent, 
if they ever occur in the normal individual. Hence if 
any eye movement occurred during the measurement 
of the interpupillary distance, the measured value 
would tend to be narrower than the true value. 

These results are recorded in a Report to the Ser- 
vices from Section D-2, NDRC. (9) This report makes 
the following recommendations: that it is important 
(a) to exercise extreme care in obtaining the correct 
measurement of the interpupillary distance of each 
height or range finder operator before he begins his 
training; and (b) to set this measured distance care- 
fully into the range finder. 

How important this correct interpupillary adjust- 
ment may be is pointed out in several analyses made 
by the Princeton Laboratory at Fort Monroe. In a 
theoretical study of the Ml Height Finder it is 
pointed out that the detailed behavior of a stereo- 
scopic range finder may be efficiently and simply 
analyzed in terms of points of view and reticle images. 
(515) This is done by imaging the reticles into the 
target space, and by analyzing the cases when two ob- 
jects, or an object and a reticle, are seen in monocular 
alignment. The points of view, or the points of per- 
spective, from which the judgments of alignment 
seem to be made, are found to be the centers of the 
entrance pupils of the system. An additional theo- 
retical study (518) shows the change of separation of 
the points of view of an Ml Height Finder as inter- 
ocular setting is varied for three sizes of observer’s 
pupil and for full and 1-inch aperture stop on the 
end-windows. It is emphasized that the edge of the 
pupil of the observer acts as a second stop and hence 
centering of the pupil of the observer to the exit 
pupil of the instrument must be exact if the freedom 
of movement is not to be constricted. 

The Princeton Laboratory at Fort Monroe also 
reported an experimental study on the effect of inter- 
ocular setting changes on range and RCS settings. 
(521) Two expert observers were used. Test was made 
with a standard Ml Height Finder at nine inter- 
ocular settings, with 12 and 24 powers and with three 
end-window stop conditions from full to 1-inch 
aperture. It was found that change of interocular 
setting produces a proportional change in range 
reading of considerable magnitude. The amount of 
shift is reduced to one-sixth with 24x magnification, 
or to one-third with 12x by a 1-inch stop either at the 
end-window or at the objective. 

Tufts College also reported another experiment on 


effects of interocular settings on ranging errors. 

(561) These experiments were again made with the 
Navy Mark 2 Stereo Trainer and show that constant 
error is a linear function of the inter-eyepiece dis- 
tance separation used. An error of 1 mm in the set- 
ting of the scale of the instrument produced a change 
in constant error of ranges of from 1 to 2 UOE. These 
errors are negative when the separation is too small 
and positive when the separations are too large. In 
the report is also the description of a device for meas- 
uring inter-eyepiece distance which was not further 
developed. A table of measurements of 10 subjects 
using a millimeter rule, the Shuron apparatus, the 
Navy Trainer, and this new apparatus shows de- 
creasing variability in that order. 

The preliminary observations noted above led to a 
serious study of interocular adjustments in stereo- 
scopic range finders and their effects. The reports on 
this subject are discussed in a Report to the Services 
issued by the Fire Control Section of NDRC to which 
the reports of original studies are attached as sup- 
porting evidence. (38) 

It had already been pointed out in a Report to 
the Services (18) that serious range errors can occur 
unless the distance between the oculars of the range 
finder accurately corresponds to the interpupillary 
distance of the observer. The errors in question can 
be very materially reduced by the use of suitable 
diaphragms and by care in maintaining the focal 
adjustment of the objectives. In this report it was 
recommended, so far as the present problem is con- 
cerned, that an accuracy requirement of 0.25 mm be 
placed upon the interocular adjustment mechanism 
and that it be required to be self-locking. Also it was 
noted that no entirely satisfactory means for meas- 
uring the observer’s interpupillary distance was then 
available and that NDRC was in process of develop- 
ing such a measuring instrument. 

This instrument was developed by the Harvard 
Fatigue Laboratory and is described in the report. 
(282) This new instrument consisted of a plane 
mirror at a distance of 33 cm before the subject’s 
eyes. Two stadia wires were mounted 3.5 cm in front 
of the mirror on the graduated limb of a microscope 
stage. The lateral separation between the two wires 
was adjusted by the observer. The position of the 
right wire was controlled by a coarse rack-and-pinion 
adjustment, that of the left wire independently by a 
fine adjustment. The observer’s head was steadied 
by a Bausch and Lomb head support. In operation, 
the observer fixated with one eye the reflected image 


riUESTRICTEDX 

^ , .. . , j ^ 


THE INTERAXIAL DISTANCE 


31 


of that eye and then moved the proper stadia wire 
until it appeared to bisect the reflected image of the 
observed pupil. This procedure was repeated for the 
other eye. When the observer had checked his set- 
tings for each eye and was satisfied with the adjust- 
ments, the distance separating the two stadia wires 
was read from the scale and vernier to determine the 
interpupillary distance. 

The report also contains a discussion of the possi- 
ble sources of error of this and other available instru- 
ments for measuring the interpupillary distance. 
Experiments were completed using the Zeiss Gauge, 
the Shuron Gauge, the Shuron Gauge with mirrored 
image, the Bausch and Lomb Gauge, and the new 
instrument. Only the new Harvard instrument and 
the Bausch and Lomb Duplex P.D. Gauge provide 
measurements of sufficient reliability for the purpose 
at hand. Both of these give sufficiently accurate and 
repeatable readings. However, their readings do not 
agree, and it has been necessary to determine which, 
if either, measures the optimum distance to set into 
a range finder. 

4.3.2 Measurement of Interaxial Distance 

During the Harvard experiments (282), while 
measuring IPD with the mirror and stadia instru- 
ment, the observers frequently reported parallax 
between the stadia wires and their reflected images. 
These reports are noteworthy because the parallax 
was perceived when the real wires appeared to bisect 
the observer’s pupils. This state of affairs suggested 
that the observer’s external lines of sight did not 
pierce the geometrical centers of their natural pupils. 
The suggestion seemed important because, if true, 
it meant that IPD is an inaccurate measure of the 
distance separating corresponding lines of sight in 
the two eyes. Practically all of a number of observers, 
both experienced and naive, noted the differential 
effect in varying degrees. Hence the Harvard group 
began to make a new set of measurements, namely 
the separation between the stadia when the parallax 
between each wire and its reflected image was re- 
duced to zero. This measurement was called inter- 
axial distance [lAD] and presumably measures the 
distance between the two principal visual axes of 
the eyes. 

For four skilled observers the difference between 
the IPD and the lAD ranges from 0.06 to 0.77 mm 
and in three of the four cases the I AD was larger. For 


14 other subjects the lAD was larger in ten cases, IPD 
larger in two cases, and not significantly different in 
the remaining two subjects. These observations could 
be repeated with only very slight variability. 

A study of IPD and lAD separations was then 
made by the Harvard Group on 24 enlisted men of 
the Coast Artillery Antiaircraft Battalion 605, Bat- 
tery C. The mean variation for each set of readings 
was, on the average, of the order of 0.2 mm. The 
average I AD value for the group was 65.07 mm. 
Thus, the difference between the lAD and the IPD 
of the group measured with the same instrument was 
0.85 mm. Very few of the subjects gave the same 
values for both measurements. Nearly all of the sub- 
jects had the I AD greater than the IPD. The similar 
testing of 105 servicemen at Camp Davis gave similar 
results. In both groups extreme cases were found in 
which the lAD was 4 mm larger than the IPD. These 
results support the hypothesis that the lAD does not 
equal the IPD in observers with normal vision in 
most cases. In other words, the principal visual axis 
of the eyes seldom corresponds to the geometrical 
center of the pupil but is usually found on the tem- 
poral side of these points. 

As a result of these findings the Bureau of Naval 
Ordnance contracted with the American Cyanamid 
Company to construct several experimental models 
of the lAD gauge which was named the interaxial- 
ometer. The completed instruments gave determina- 
tions with excellent precision and consistency. 

The Harvard Group then made a study of the 
properties of the interaxial distance. They found 
that the measured lAD values were very nearly the 
same for each of four accommodated distances vary- 
ing from 33 cm to infinity and that lAD remains 
constant for verged distances from 0 to 8 degrees or 
uncontrolled vergence. Although there is a consider- 
able difference in IPD with lateral rotation of the 
eyes, none was found for I AD under these conditions 
or for vertical rotary eye movements or vertical rotary 
head movements. On the basis of these results, the 
investigators present the hypothesis that within wide 
limits each human eye may be treated as if it possesses 
but one perspective center, whether the eye is sta- 
tionary or moving, and that this center lies in the 
vicinity of the center of rotation of the eye. The prac- 
tical significance of this idea is that it permits the 
eye to rotate through a wide angle without occasion- 
ing a perceptible amount of parallax between any 
two fixed sights. The measurements demonstrated 
that when the eyes are rotated through angles of 


j jlEsTRK.n rq 


32 


PERSPECTIVE ERROR 


±8 degrees, the distance separating corresponding 
lines of sight in the two eyes was constant. 

The Harvard report contains a description of a 
self-locking interocular device for controlling the 
separation between eyepieces in the Ml Height 
Finder and the Mark 42 Range Finder. This idea 
was adopted by the Services and is important because 
it was discovered, in the previous non-locking ar- 
rangement, that with wear there was a tendency for 
the eyepieces to spread when the operator pressed his 
eyes into the eyecups. The report also outlines the 
method to be employed for calibrations of the 
axialometer. 

In the final section of the Harvard report, the re- 
sults of experiments are given to indicate whether 
I PD or I AD, if either, should be used to separate the 
oculars in balanced stereoscopic range finders. The 
indication seems fairly clear and conclusive. Per- 
spective parallax occurred when the lines of sight 
of the oculars were separated by the I PD of observers 
whose IPD was not equal to their lAD. The amount 
of this perspective parallax, by measurement, was 
found to be proportional to the difference between 
the IPD and the I AD of the observer. When the 
oculars were separated by an amount equal to the 
observer’s lAD, on the other hand, perspective paral- 
lax was eliminated. The use of reduced apertures 
is another method for minimizing the effects of per- 
spective parallax in stereoscopic range instruments 
and experimental results are presented to demon- 
strate that when the exit pupils of the instrument 
are smaller than the natural pupils, the observer’s 
lines of sight can in effect be made to coincide and 
move with the lines of sight of the instrument. Paral- 
lactic errors produced by asymmetrical “clipping” 
are emphasized when the instrument is stopped down 
and it is believed that the likelihood of occurrence 
of these “clipping” errors should be alleviated by 
separating the reduced exit pupils by a distance 
equal to the observer’s IPD. 

Incidental to a Navy Bureau of Ordnance report 
on the effects of helium charging on range accuracy, 
it is pointed out that such accuracy, with improper 
interpupillary settings in air and in helium, are of 
such magnitude as to obscure the actual effects of 
helium which are considerable. (318) 

4.3.3 Effect of Perspective Error 

A study of the effects of interocular setting of range 
finders having perspective error is reported by the 


Bausch and Lomb Optical Company. (107) Settings 
for interocular distance were used with the instru- 
ment full open or stopped down. The interocular 
distance was varied systematically in an effort to 
determine whether or not the optimal ranging re- 
sults most closely approximated the IPD or the I AD 
measurements. In this investigation an improper ad- 
justment of a known amount was deliberately intro- 
duced into a Mark 58 Range Finder which viewed 
through an indoor collimator. Four expert observers 
were used. The results of one observer showed the 
interaxial distance to be the correct setting. Two 
other observers show a setting which is neither the 
interaxial distance nor the interpupillary distance, 
but is closer to the former. The interaxial distance 
of a fourth observer was not known. It will also be 
noted that for the first three observers the lAD was 
narrower than the IPD, which is contrary to the 
average situation as found in the measurement of 
these distances in a considerable number of indi- 
viduals by the Harvard group. The relation of these 
results to theory is considered. In conclusion it is 
pointed out that the unavoidable sources of error 
are small enough to make an exact choice of the 
interocular setting of secondary importance. This, 
however, is by no means true of an instrument that 
is not properly adjusted. The specifications as usually 
written now permit a misadjustment for perspective 
error of 0.2 of a diopter. In view of the findings of 
this experiment it is obvious that this is a much 
larger error than should be tolerated. Since it is not 
a particularly difficult adjustment to make, it is high- 
ly desirable to specify that all range finders should 
be adjusted to eliminate perspective error for some 
prescribed object distance for the main system. A 
similar adjustment should be required for the in- 
ternal adjuster system. 

Some experimental results are reported from 
Camp Davis through Brown University using an Ml 
Height Finder. (150) The target was a telephone 
pole at 1,700 yards. Ten observers of a stereoscopic 
observers’ class were used as subjects. Three subjects 
had a difference of 0.25 mm between their lAD and 
IPD, three a difference of 0.5 mm, and four a differ- 
ence of 0.75 mm. The results indicate, in terms of 
these differences, that for this group of men change 
in consistency was not related to the amount of 
difference between the interaxial and interpupillary 
distance. 

Brown University made a report which is also 
included in this material reported to the Services. 


RECOMMENDATIONS 


33 


(149) Because it had been shown that it is necessary 
that the range finder observer adjust the separation 
of the oculars on the instrument so that the distance 
between the two exit pupils is exactly the same as his 
interocular distance, and because it was discovered 
that the interpupillary distance scales mounted on 
the height and range finder eyepiece assembly are 
not sufficiently accurate to guarantee a precise setting 
of the separation of the two exit pupils, some other 
method had to be employed. This memorandum 
describes the construction of a template for use in 
making precise settings of ocular separation once 
the observer knows his interocular distance. The 
template is easy to construct and should be provided 
for each observer. If the observer uses the template 
in the manner described in the report, he can adjust 
the exit pupil separation on the range finder so that 
it is not in error by more than 0.25 mm. The template 
consists merely of a strip of hot rolled steel. Two 
small holes are drilled exactly the desired interpupil- 
lary distance separating their two centers. In each 
case the size of the holes exactly corresponds to the 
size of the exit pupil of the instrument. The exact 
interpupillary separation to 0.25 mm is stamped on 
each template. The holes are covered with tissue 
paper. 

The method of using the template is: (1) focus 
both eyepieces to the same value; (2) point the in- 
strument to a bright part of the sky or to the moon 
at night; (3) have an assistant narrow down the 
separation of the oculars to some value about 2 mm 
below the observer’s interocular distance. Then have 
him widen the separation by 0.25 mm steps according 
to the marks on the instrument scale, stopping as 
soon as the two exit pupils are seen exactly centered 
in the two holes of the template. The settings should 
always be started at less than the interocular distance 
to minimize the backlash error. If end-window stops 
are being used instead of the full-field instrument, 
the instrument should be set for range and the meas- 
uring scale adjusted to read 5,000 yards before mak- 
ing observations of the exit pupil separation. 

Recommendations 

On the basis of this considerable mass of data the 
following summary and recommendations are made 
in the covering Report to the Services. (38) 

1. When the exit pupils are very small (about 
1 mm) the errors due to errors of interocular setting 
are negligible. 


2. As the exit pupils are made larger, the error 
gradually increases, reaching its maximum value 
when the exit pupil is about the size of the pupil 
of the observer’s eye. 

3. WTen the exit pupil is larger than the pupil of 
the observer’s eye, the error is independent of the 
size of the exit pupil. In some cases it has been found 
to reach its theoretical value, but in the careful study 
at Bausch and Tomb the theoretical value was ap- 
proached only at low magnifications. No explanation 
has been found for this apparent dependence of 
perspective error on magnification. 

4. The interocular setting which gives zero per- 
spective error is not critical with small exit pupils. 
'Whth large exit pupils it is critical and, in the case 
of some observers at least, equal to the interaxial 
rather than the interpupillary separation. But even 
under carefully controlled conditions there are other 
factors which affect the true range reading fully as 
much as the difference between interaxial and inter- 
pupillary distance; e.g., if the target is not kept in 
the same position of the field in which the internal 
adjuster target was set systematic errors may be intro- 
duced which will completely mask those due to a 
moderate amount of interocular maladjustment. 
The operator cannot be expected to use interpupil- 
lary distance under some conditions and interaxial 
separation under other conditions. Therefore some 
sort of a compromise must be effected. 

In view of all these considerations, it seems evident 
that the elimination of perspective errors is a matter 
of extreme difficulty if exit pupils larger than the 
pupil of the eye are used. Therefore the instruments 
should be stopped down wherever possible. In this 
stopped-down condition, the oculars should be set 
to the interpupillary distance, since maximum head 
freedom is thus achieved. In conditions of dim light, 
it will be necessary to remove the stops in order to 
gain light-gathering power. It may also be necessary 
to get additional head freedom by increasing the 
exit pupil size, in the Naval situation, when a ship 
is rolling badly. But under either of these conditions 
it will be impossible to obtain ranges of high pre- 
cision, no matter what interocular setting is used. 
Presumably the interaxial setting would be the opti- 
mum, but in the presence of other large sources of 
error the gain to be expected is not great enough to 
warrant the adoption of a special practice. Hence it 
should be concluded that the interpupillary distance 
should be used in all cases. 

This reasoning leads to the following recom- 


RESTRIC7Ei7\ 


PERSPECTIVE ERROR 




iiieiidaiions made to the Services. (1) The observer’s 
iiiterpupillary distance shoidd be measured by means 
of the Baiisch and Lomb Duplex P.D. Gauge, or its 
equivalent, and this distance should be set into the 
oculars at all times. (2) For instruments whose inter- 
ocular scales cannot be trusted, the oculars should be 
set by means of templates described above. This 
procedure should, however, be regarded only as a 
stopgap until such time as the more accurate inter- 
ocular mechanisms described in the previous NDRC 
Report to the Services (18) are available. These rec- 
ommendations should emphasize that the instru- 
ment shoidd be stopped down whenever operating 
conditions permit. 

The Applied Psychology Panel subsequently re- 
ported two direct methods of putting the correct 
interocular setting into the range finding instru- 
ments, avoiding the use of a template or interpu- 
pillometer which involves special equipment. (75) 
Neither method proved to be either accurate or re- 
liable. It is recommended that adequate interpupillo- 
meters for measurement of interpupillary distance 
and templates for each observer be provided for use 
in setting the oculars of all height and range finders. 

A final report on this topic was made by the Ap- 
plied Psychology Panel regarding the precision of 
setting the oculars of the range finder with inter- 
pupillary distance templates of different design. (78) 
This is an adjustable template developed by the 
Naval Bureau of Ordnance which is described in the 
report. This report describes ten settings of the ocu- 
lar of a range finder by each of 10 students at the 
Fort Lauderdale Naval Training School, Fire Con- 
trolmen (R) with the Bureau of Ordnance template 
and holder. Five of these settings were made with a 
reticle pattern consisting of two pairs of vertical 
parallel lines in the template. The other five settings 
were made with a reticle pattern consisting of two 
small circles. In addition, these men made five set- 
tings of the range finder oculars with their own per- 
sonal NDRC templates. 

It was found that there was no statistically sig- 
nificant difference in the precision of setting the 
ocular separation between any of the three template 


conditions. In all cases, the precision of the settings 
was better than 0.25 mm. However, the Bureau of 
Ordnance variable template has advantages over the 
NDRC fixed template in that it is more convenient 
and allows a quicker adjustment; its holder places its 
reticles in the plane of most distinct focus of the light 
emerging from the exit pupils; and it can be made 
suitable for any range finder operator, thus avoiding 
the necessity of each operator’s having his personal 
template available at all times. It was also found that 
the circle reticle pattern has the disadvantage with 
some range finders of having the small circles off- 
center along the vertical axis of the exit pupils. The 
parallel line reticle does not have this disadvantage. 
The holder provided fits the template to the oculars 
of Range Finders Mark 42, models 4 to 6 and 8 to 13 
only. However the Bureau of Ordnance templates 
can be used on other range finders without the 
holder. When this is done, the same procedure 
should be used as that developed for the use of the 
NDRC template noted above. The use of this type 
of template and the development of holders for other 
types of range finder is recommended. 

A Frankford Arsenal Princeton Branch report 
presents specifications and outlines a design for an 
instrument to be incorporated in the Ml Height 
Finder and to be used for measuring the distance 
between the exit pupils of the instrument with an 
error of less than 0.1 mm. (230) The instrument is 
also to be used for setting precisely a prescribed dis- 
tance between the exit pupils, such as the observer’s 
interpupillary distance. Such an interpupillometer 
consists of two concentrically marked, translucent 
reticles so mounted in the plane of the exit pupils, 
with a reticle in front of each exit pupil, that each 
reticle may be moved in the plane of the eyepiece 
assembly plate, both parallel and perpendicular to 
the axis of the height finder. When the reticles are so 
adjusted that each exit pupil is concentric with the 
concentric markings of its reticle, the distance be- 
tween the exit pupil centers may be read from a 
calibrated micrometer which provides and measures 
the movement between the reticles in the direction 
of the height finder axis. 


Chapter 5 

TEMPERATURE EFFECTS 


5 1 THERMAL EFFECTS IN COINCIDENCE 
RANGE FINDERS 

T he British discovered late in the 30 ’s that range 
finders tend to develop errors on elevation that 
are not present prior to elevation. The errors re- 
vealed appear to be quite definite, but no systematic 
investigation had been carried out under Service con- 
ditions and the atmospheric conditions most favor- 
able to the development of these errors were not 
known. Towards the end of 1938 the Admiralty Re- 
search Laboratory [ARL] decided to investigate the 
problem and hence apparatus was designed which 
would enable the errors of range finders mounted in 
the open to be examined at any time during the day 
or night, at any angle of elevation, and without the 
necessity of utilizing any external object to serve as a 
ranging mark. Furthermore, the British found that 
wartime experience of the results of antiaircraft gun- 
nery provided its quota of evidence that range find- 
ers tend to read low on elevation and ARL was asked 
in 1939 to investigate the problem without waiting 
for the delivery of precise testing apparatus. At this 
time the daily variation of the coincidence adjust- 
ment at zero angle of elevation of two Type F.Q. 25 
range finders had been under investigation at ARL 
Hence it was decided to prepare a number of reflect- 
ing windows and to use them immediately in con- 
junction with these two range finders. The first 
report from ARL deals with these findings. (56) 
Before this time it had not been practicable for 
two reasons to make use of the sun as a ranging mark. 
In the first place, there is the risk of damage to the 
eye of the observer, and although this objection can 
be met by providing a dense neutral filter over the 
eyepiece, such filters would be likely to crack owing 
to the concentration of heat in this position. A far 
more serious objection lies in the fact that the whole 
of the heat transmitted into the instrument is avail- 
able for disturbing various balsamed surfaces and, 
more particularly, for upsetting the general thermal 
conditions previously existing in the instrument. It 
appeared that these objections could be met by 
mounting partially reflecting plane parallel windows 
directly in front of the existing height finder win- 
dows. The metallic deposit on these additional win- 


dows would reflect back all the heat likely to create 
disturbing temperature gradients within the instru- 
ment, and by suitably controlling the density of the 
deposit, sufficient light might be transmitted to per- 
mit a clear view of the sun. A number of these re- 
flecting windows were accordingly made, checked for 
plane parallelism, and were used throughout the 
tests whenever observations at the elevation of the 
sun were required. They were satisfactory for this 
experimental purpose and proved that a transmis- 
sion of 0.006 per cent of the light was acceptable. 

Experiments involving systematic observations of 
elevation error were carried on both by day and 
night. By day, the ranging marks employed were the 
sun and moon whenever a\ ailable, together with a 
church spire of known range to give the error at zero 
angle of elevation. By night the stars and moon were 
used as targets. At first ARL personnel acted as ob- 
servers but subsequently six Service range takers 
were made available for this purpose. 

The standard procedure was to obtain sets of 
values of the error at various angles of elevation and 
at approximately hourly intervals during the com- 
plete period of 24 hours. Internal adjuster readings 
were obtained for each set of readings. A single read- 
ing in the report is the mean of ten separate observa- 
tions of the error expressed in seconds in the object 
space. During months prior to the experiment, Barr 
and Stroud had been fitting internal temperature 
tubes to new deliveries of range finders of Type 
F.Q. 25. The laboratory investigation was therefore 
directed in its initial stages to an examination of the 
error to be found in instruments both with and with- 
out these tubes. 

A study of the results leads to the following con- 
clusions. (1) The reflecting windows proved satis- 
factory in that their use does not modify in any way 
the previously existing error of the range finder. 
Hence the safe use of such windows for a field check 
of the coincidence adjustment on the basis of solar 
observations seems confirmed. (2) Elevation errors 
are not constant in amount throughout the day, but 
exhibit a very marked tendency to disappear in the 
region of 6 a.m. to 8 a.m. and again between 5 p.m. 
and 7 p.m. On the other hand, they tend to rise to 
a maximum between 1 1 a.m. and 1 p.m., and between 


•RF-TRiLTF' 


35 


36 


TEMPERATURE EFFECTS 


10 p.m. and midnight. These hours, it is stated, are 
only approximate and, no doubt, in any particular 
case they will be closely connected with the variation 
of external temperature, and the incidence of direct 
radiation from the sun, throughout the day. There 
is evidence that the periods of zero error occur rough- 
ly 2 hours after the times of stationary external tem- 
perature; those of maximum error, 2 hours or so after 
those times at which the rate of change of external 
temperature is greatest. (3) Elevation errors are in- 
variably negative, i.e. the instrument if adjusted cor- 
rectly at zero elevation reads low at higher elevations. 
For the F.Q. 25 type of range finder it was found that 
the maximum elevation error may be considerable 
and at times errors amounting to 13 seconds of arc 
were shown. (4) It was also found that the presence 
of internal temperature tubes gives limited improved 
performance. 

Temperature Tubes and Air-Stirring 

The investigators suspected that the cause of the 
elevation was to be found in the presence, in the air 
within the tube of the range finder, of certain tem- 
perature gradients which themselves give rise to devi- 
ations in the path of the light. In other words, 
radiant heat from the sun on the top of the tube 
would set up temperature stratification of the air 
within the tube of the instrument. A positive vertical 
temperature gradient within the tube would produce 
a systematic tendency to low readings at the higher 
elevation. Such gradients, no^ doubt, can be very 
small and might be difficult to record experimentally. 
It should be remembered, however, that the total 
light path within the range finder investigated is of 
the order of 18 feet, and it may well be the case that 
the cumulative effect of these small gradients over 
such a comparatively long distance may be appreci- 
able when reduced to seconds. The introduction by 
Barr and Stroud of internal temperature tubes which, 
it was hoped, would promote temperature equaliza- 
tion, was based on this theory of the origin of the 
error. These tubes have met with a measure of suc- 
cess at times. At other times, however, and particu- 
larly by day, the residual error after fitting with 
internal tubes is by no means negligible. For ex- 
ample, in one case, an instrument so filled revealed 
an error on the sun of 9.8 seconds at an elevation of 
50 degrees. 

Because the temperature tubes did not give com- 


plete satisfaction in eliminating elevation errors 
other solutions were considered. It is not improbable 
that the substitution of internal vacuum tubes for 
the existing temperature tubes would yield some 
improvement in performance. A comparatively low 
vacuum should suffice for this purpose, but such a 
modification could not readily be adapted to existing 
instruments in Service. Therefore it was decided to 
experiment with a simpler solution and one which, 
if successful, could be rapidly fitted. The effort was 
made to equalize temperatures along the light path 
between the end penta prisms and the object glasses 
carried by the inner frame by a vigorous mechanical 
stirring of the inner air. ^Vith this end in view, a 
simple reciprocating hand pump was fitted to the 
range finder in such a way that the internal air was 
agitated but no fresh air from the atmosphere was 
introduced at any stage into the range finder. With 
such a closed system, no dust, moist external air, or 
air at a temperature sensibly different from that 
within the instrument can enter. Experimental stir- 
ring seems to remove the elevation error almost en- 
tirely. Replacing the hand pump with a motor-driven 
pump indicated that continuously circulating air, as 
distinct from the type of air surges realized with the 
hand pump, was equally effective in removing the 
elevation error to a considerable extent. It was de- 
termined that such air circulation did not produce 
an optical shimmer which would affect operator’s 
consistency of readings. Averages of elevation errors 
of the range finder are: without internal tubes and 
with no air stirring, —7.6 seconds (range: —4.0 to 
— 13.3 seconds); with internal tubes, —4.0 seconds 
(range: -f-2.7 to — 1.10 seconds) and without tubes 
but with stirring, —0.3 seconds (range: -|-2.3 to 
—3.4 seconds). The elevation errors of the internal 
adjuster were: without tubes or air stirring -\-b.l 
seconds (range: -j-10.6 to -1-0.5 seconds); with in- 
ternal tubes —0.2 seconds (range: -(-7.3 to —6.0 
seconds) and without tubes but with stirring —0.4 
seconds (range: -)-4.0 to —3.4 seconds). Hence these 
results show that the introduction of internal tem- 
perature tubes reduces the mean elevation error of 
the range finder from about —7.5 seconds to —4 
seconds. The method of stirring, however, substan- 
tially eliminates the error, the small residual being 
less, on the average, than 1 second. 

For the stirring procedure, it was realized that, on 
stopping the pump, the error would reestablish it- 
self in course of time. When the original experiments 


'RESTRICTFDA 


STRATIFICATION AND AIR STIRRING 


37 


were carried out on the sun it was noticed that, as 
soon as pumping was stopped, the coincidence cut 
broke continuously at such a rate that it became 
impossible for the observer to maintain the cut dur- 
ing this period. Hence a continuous photographic 
record of the appearance of the cut was taken. Before 
the stirring commenced the mean error was —10 
seconds, but within 7 seconds of the hand pump 
being started the error had changed to -f 1 second 
at which value it remained during the remainder of 
the stirring. On stopping the pump the error re- 
mained steady for about 10 seconds, after which a 
further change, occupying 10 seconds of time, to -(-5 
seconds of arc took place, followed gradually by the 
return, after a further 20 seconds to its original value 
before stirring commenced. Thus the error was 
largely eliminated very quickly after pumping was 
commenced but returned to its maximum value with- 
in 40 seconds of stopping of the pump. No explana- 
tion was advanced for the temporary alteration in the 
error, amounting to -\-4 seconds, which occurred 
20 seconds after pumping was stopped. 

Similar experiments were carried on and simulta- 
neously reported by ARL on the elevation errors of 
range finders Types U.D. 4 and U.K. 4. (55) The find- 
ings substantiated those already outlined above for 
the Army instrument. Type F.Q. 25 for this Navy 
instrument. It was found that the elevation error is 
not constant but varies rhythmically throughout the 
day, rising to a flat maximum around 1000 to 1600 
hours. In all cases, the elevation error tends to indi- 
cate low ranges. The maximum error observed was 
11 seconds in an instrument with base length of 15 
feet, hence an error of this magnitude corresponds 
to a range error of 275 yards at 5,000 yards. 

The U.D. 4 instrument is normally provided with 
two desiccating tubes leading outwards to the re- 
spective ends of the range finder from two connec- 
tions near the eyepiece. With a view to facilitating 
any instrumental modification which ships might be 
called upon to undertake, it was first decided to make 
use of these existing tubes for stirring the interval 
air. Such an arrangement made little impression on 
the elevation error and the desiccating tubes proved 
to have leakages. Hence new holes were provided in 
the range finder in a way described in the text. Such 
stirring of air substantially reduced the elevation 
errors in U.D. 4 but not to the same extent as with 
the F.Q. 25 instrument. This is probably due to 
differences in design and, it will be remembered, that 


the improvised stirring relates only to the space be- 
tween the two ends of the inner frame and the end 
reflectors. It is possible, therefore, that the small 
residual errors not removed by the fitted jet tubes 
may be attributed to temperature gradients within 
the inner frame. Also, the investigators point out that 
small residual mechanical bending effects should not 
be overlooked. For this instrument the mean eleva- 
tion error at 30 to 40 degrees elevation was standard: 
—4.9 seconds (range: —3 to —11 seconds); fitted 
with jet tubes: —1.3 seconds (range: -^1 to —4 
seconds). However, the weather was cooler and more 
unsettled during these tests than during those for 
F.Q. 25, with the result that no large errors were 
developed in the instrument in its standard condi- 
tion. Results show that the 12-foot U.K. 4 Range 
Finder is susceptible to elevation errors of the same 
order of magnitude as the longer 15-foot U.D. 4 in- 
strument and again that internal stirring effects a 
considerable improvement in performance in this 
respect. 

During the autumn and early winter of 1940, ARL 
made a further comparative study of the elevation 
errors of the Goerz Range Finder No. 8 and the Barr 
and Stroud No. 10 instrument by the same experi- 
mental methods. (57) These experiments were car- 
ried on in October and November when the British 
weather is cool and more even temperatures may be 
expected. It was found that during this autumn 
period the average elevation error of the Goerz in- 
strument was about three times that of the Barr and 
Stroud, the latter being about one-half that obtained 
with the same instrument during the summer period 
reported above. Internal stirring of the air reduced 
the mean elevation error of the Goerz instrument 
from —3.9 seconds (range: +1.1 to —9.6 seconds) 
to —1.7 seconds (range: +2.6 to —5.1 seconds) and 
for the Barr and Stroud from —1.1 seconds (range: 
+0.7 to —3.9 seconds) to +0.5 seconds (range: 
+2.8 to —3.2 seconds). 

Finally, during this period, ARL investigated the 
elevation errors of Army Range Finder No. 3 Type 
U.B. 7. (58) The mean altitude of the sun during 
these trials was only 31 to 38 degrees. When not 
fitted with internal temperature tubes and with no 
air stirring the mean elevation error was —4.6 sec- 
onds (range: —2.0 to —8.4 seconds); with air stir- 
ring but with temperature tubes removed —1.5 sec- 
onds (range: +2.2 to —5.3 seconds) and fitted with 
temperature tubes —1.2 seconds (range: +1.9 to 


rRESlRlCTE] 


38 


TEMPERATURE EFFECTS 


—7.1 seconds). The investigators conclude from 
these results that: (1) The fitting of internal tem- 
perature tubes reduces previously existing elevation 
errors by about one-half. (2) Air disturbance by the 
operation of jet tubes is a more effective means of 
reducing elevation errors than is provided by the 
fitting of internal temperature tubes. On the average 
these jet tubes reduce the residual elevation error to 
a value of about —1 second. (3) Thermograph rec- 
ords taken continuously during the trials indicate 
that for these instruments within the range of tem- 
peratures from 10 C to 15 C the variation of co- 
incidence adjustment at zero angle of sight is not 
greater than 0.25 second per degree centigrade. 

5 2 THERMAL EFFECTS IN STEREOSCOPIC 
RANGE FINDERS 

Charging with Helium 

It will be noted that these earlier experimental 
observations on elevation error and temperature 
effects were made on range finders of the coin- 
cidence type. The Princeton Laboratory at Fort 
Monroe first studied these thermal effects in the 
stereoscopic type instruments. (345) In theory, de- 
veloped in the report and given at considerable 
length as an appendix, vertical temperature gradi- 
ents along the optical paths of a height finder will 
produce refraction of the light rays in a vertical 
plane. If such stratification exists, when the instru- 
ment is elevated, this refraction will produce an 
effect on the plane of triangulation proportional to 
the sine of the angle of elevation. With a temperature 
gradient of 1 F per inch in the Ml 13 1 /2-foot Height 
Finder, this effect amounts to 40 seconds total angu- 
lar refraction over a path length of 29 feet if the 
instrument is charged with dry air or nitrogen. Parts 
of this total refractive angle compensate each other 
leading to a smaller error which depends on the 
angle of elevation at which the internal adjuster 
readings and range readings are made. In this case, 
the range error in reading on an aerial target at 4,000 
yards altitude and true slant range 8,000 yards is 23.3 
UOE if the internal adjuster is read at the same 
angle. The magnitude of the stratification effect 
depends on the refractive index of the gas and is 
directly proportional to the excess of the refractive 
index over unity. For nitrogen or dry air this excess 
is 29 X 10-5. For helium it is 3.6 x lO'^. The stratifica- 


tion effect, assuming equal temperature gradients 
and equal gas pressures, would therefore be reduced 
to about 12 per cent of its previous value by the sub- 
stitution of helium for nitrogen. The stratification 
effect with a gas mixture cannot be obtained with 
complete accuracy by linear interpolation based on 
percentage composition because of an additional 
effect due to thermal diffusion. But it is estimated 
that a mixture of 95 per cent helium and 5 per cent 
nitrogen would reduce the stratification effect to 
about 19 per cent of its value for an instrument 
charged with pure nitrogen. 

The Princeton Laboratory then proceeded to test 
this theory experimentally, using the internal ad- 
juster system of the Ml Height Finder. To accom- 
plish this it is necessary only to produce a uniform 
vertical temperature gradient within the main tube 
and determine the difference between the internal 
adjuster settings taken, hrst, with the plane of tri- 
angulation vertical and, second, with the plane of 
triangulation horizontal. The size of this difference 
depends only on the magnitude of the temperature 
gradient and on the refractive index of the gas with 
which the instrument is charged. Inasmuch as facili- 
ties were not available for the creation of extreme 
or uniform temperature gradients, an alternative 
method was employed. This consisted of using a 
height finder soon after it was moved from the shade 
and exposed to the sun. Thermocouples were placed 
at various points within the instrument and it was 
found that differences in gas temperature between 
the top and bottom of the main tube as great as 1 F 
per inch are sometimes produced by this exposure 
to the sun. Thermocouple measurements have also 
shown that the temperature gradients resulting from 
this exposure are not constant along the axis of the 
main tube and that the temperature distribution 
within the instrument is otherwise not uniform. For 
example, gas temperature differences as great as 4 F 
have been observed between the two ends of the in- 
strument, and smaller differences of metal tempera- 
tures have been found to exist between the ends of 
the optical bar. 

It is also possible that differences in internal ad- 
juster settings for the two positions of the plane of 
triangulation may be due, in part, to factors other 
than temperature stratification. From the data of 
this study there is strong evidence that not all instru- 
ments behave in the same way. This type of variation, 
which may be due to differences in construction. 


RESTIUCTE] 


MEASUREMENTS OF STRATIFICATION 


39 


makes difficult the isolation of that part of the ob- 
served differences in internal adjuster readings due 
only to temperature stratification. It is likewise possi- 
ble the observers vary in their reading of the internal 
adjuster as the instrument is elevated or depressed, 
due perhaps to the change in head position. In spite 
of these difficulties, the present experiment estab- 
lished qualitatively that the theoretical stratification 
errors occur as predicted. Further work was needed 
for quantitative verification. As shown by the strati- 
fication theory, the magnitude of the effect of eleva- 
tion on internal adjuster readings is determined, in 
part, by the refractive index of the gas with which 
the instrument is charged. This fact makes possible 
a further experimental verification which was ac- 
complished in this study by the use of instruments 
charged with helium and, at different times, with 
nitrogen, the effect being almost entirely eliminated 
when helium was used. Finally, the experimental 
evidence that the use of helium reduces the refractive 
errors proves the practical advantages of using this 
gas for charging the instruments in the field. For ex- 
ample, for one instrument the elevation error was 
found to be —1 1 seconds when charged with nitrogen 
and this was reduced to —2 seconds or 1 second for 
two comparable days when charged with helium, or 
a reduction to about one-eighth of the error. 

In this report a brief study was also reported of 
methods for charging the height finders. In changing 
from nitrogen to helium the two methods used suc- 
cessfully were flushing and evacuation. Evacuation is 
quite satisfactory, but under field conditions may 
be inconvenient. Experiments were, therefore, car- 
ried out on the time required for adequate flushing. 
In several cases, after flushing with 50 cubic feet of 
helium, the content of nitrogen remained well be- 
low 5 per cent. For economy of helium, flushing 
should be carried out at low rates of flow. The im- 
portant quantity is the number of cubic feet passed 
through and 50 feet seems advisable for the Ml in- 
strument. This is attainable, where tank pressure 
gauges are not accessible, by flushing for at least 45 
minutes holding at least 5 pounds gauge pressure. 

From these results the investigators conclude that 
both the theoretical and experimental evidence indi- 
cates the desirability of filling Ml height finders 
with helium since this will necessarily reduce all 
refractive errors and since there is no respect in which 
helium appears inferior to nitrogen as a charging 
gas. In order that the Ml height finder be charged 


satisfactorily with helium, it should be flushed for 
45 minutes at 5 pounds pressure to ensure that the 
final charge is at least 95 per cent helium. 

Prior to this Princeton Laboratory report, the 
same group published some preliminary results on 
this topic of temperature effects. (343) In this report 
are given the position of the thermocouples intro- 
duced into the Ml Height Finder and temperature 
trends measured by them in different parts of the 
instrument. Some preliminary results of elevation 
errors are given and helium is suggested as one means 
of reducing thermal gradients in the system. 

Princeton Laboratory at Fort Monroe has also 
presented a large number of special studies dealing 
with stratification effects. These reports give in 
greater detail the experiments summarized in their 
larger final report. (345) One report outlines the 
experimental determinations of the extent to which 
internal adjuster readings are modified by elevation 
and is aimed to discover how much of this effect 
can be attributed to long exposure to the sun. (430) 
Another considers the best method of making the 
RCS setting for these experiments. (431) The next 
report outlines the temperature characteristics of 
different instruments; (432) and the differences 
found by testing two M2 and two Ml instruments 
are tentatively assigned to differences in effective in- 
sulation. Still another report describes the place and 
method of installation of thermocouples in two Ml 
Height Finders. (433) Another paper deals with the 
effect of temperature on the internal adjuster setting 
when ranging on fixed ground targets. (434) With 
no change in elevation, there is shown no significant 
relation between temperature and the RCS setting 
for the Ml Height Finder over a temperature range 
of 35 degrees F. Similar plots for the M2 instrument 
show a well defined relation; for each degree Fahren- 
heit increase in temperature, RCS readings decrease 
by about 0.6 units. 

Another Princeton Laboratory report gives results 
indicating the reduction of the stratification error 
when the instruments are filled with helium as con- 
trasted with nitrogen- filled instruments. (435) An- 
other report gives thermocouple measurements of 
internal temperatures. (436) Air temperatures were 
measured at the top and bottom of the internal 
adjuster wedge cavity; and at the objective side of 
the left reticle frame. Temperatures of different parts 
of the instrument were measured at the top and bot- 
tom of the range prism cavity and the metal temper- 



40 


TEMPERATURE EFFECTS 


ature, outside of back wall of the optical bar. It was 
demonstrated by these readings that differences of 
considerable size exist between different parts of 
the instrument. For example, air temperature at the 
bottom of the internal adjuster wedge cavity might 
be considerably above the outside air temperatures 
after prolonged exposure to the sun. Also temper- 
atures at the optical bar lagged behind the more 
exposed portions of the instrument. Another report 
gives additional information regarding thermo- 
couple measurements taken on an exposed instru- 
ment throughout a day with temperatures varying 
from 8 to 100 F. (438) Another report describes the 
filling of two instruments with helium. (437) 

These studies are summarized in the Princeton 
Laboratory report described above which is attached 
as supporting data to a Report to the Services issued 
by the Fire Control Section of NDRC. (1) In this 
report attention is called to the considerable body of 
evidence indicating that ranges obtained from opti- 
cal range finders may be in error by appreciable 
amounts at times when the external temperature is 
changing or when the instruments are exposed to 
direct radiation from the sun. It was pointed out 
that temperature stratification of the gas within the 
instrument could cause such errors, and that they 
would be much reduced by filling the instrument 
with helium. The recommendation was made to the 
Services that all range and height finders intended 
for high elevation observing be charged with helium. 
This recommendation was adopted by both the 
Army and Navy for all high elevation range finders. 

Causes of Temperature Changes 

About a year later, in October 1942, the Fire Con- 
trol Section of NDRC issued another Report to the 
Services on temperature effects in stereoscopic height 
finders. (16) Five studies are attached as supporting 
evidence which will be analyzed below. 

Temperature changes may be induced in a range 
finder in a variety of ways. The temperature of its 
surroundings may change gradually, in which case 
the temperature of the instrument changes in such 
a way that at any one time all of its parts are at 
substantially the same temperature. The tempera- 
ture of the surroundings may change rapidly, in 
which case different parts of the instrument will be 


at different temperatures. Or the instrument may be 
exposed to direct sunlight, so that certain portions 
of the instrument are raised to and maintained at 
temperatures materially above those of its surround- 
ing air, and materially above other portions of the 
instrument. These causes may produce a wide variety 
of effects, either on the gas which the instrument 
contains, or upon the structure of the instrument 
itself. 

If a gas is maintained under constant pressure 
and its temperature rises, its density, and therefore 
its refractive index, decreases. This change of re- 
fractive index, if uniform, may cause slight, though 
not entirely negligible changes in the focal lengths 
of the optical systems immersed in it. For example, 
in the stereoscopic height finder, the image of the 
target, which is ideally supposed to be formed in the 
reticle plane, will change position slightly. The 
direction of shift will be along the optical axis. Since 
shifts of this kind also occur as the distance of the 
target changes and since the importance of these 
perspective errors is discussed in another report 
which is described above (18) no further mention of 
them will be made here. 

Stratification 

If the temperature is not uniform throughout the 
gas, three types of phenomena may occur. The gas 
may form fairly stable layers hot at the top and cool 
at the bottom. In this case any beam of light passing 
lengthwise throughout the instrument will be de- 
flected downward. This is the stratification effect 
discussed above. (18) The hotter parts of the gas 
may also form in stagnant pockets, the boundaries 
of which will in general be curved. Such pockets will 
act as rather weak and ill-defined lenses or wedges, 
thus both blurring the images and shifting them 
relative to the reticle. The shift in this case may be 
either along the axis or normal to it. Finally, con- 
vection currents may be set up, the curved boun- 
daries of which may affect a beam of light in exactly 
the same way as would a stagnant pocket. In addi- 
tion, these pockets transport heat from one part of 
the instrument to another and thereby cause inequal- 
ities in the temperature of its structural parts. For 
example, such a current might flow upward along one 
side of a plane mirror making that side warmer than 
the opposite face, and thereby causing curvature. 

All of the effects so far mentioned are alleviated 


EXPANSION OF STRUCTURAL PARTS 


41 


by the use of helium as a charging gas, partly because 
its refractive index differs so little from unity at all 
temperatures, and partly because its thermal con- 
ductivity and viscosity aid in the rapid elimination 
of temperature differences within the gas itself, or 
between the mechanical parts immersed in it. 

There is another and largely unrelated set of tem- 
perature effects in the gas due to its expansion and 
contraction with temperature. The instruments 
under discussion are in all cases initially sealed, but 
they are subject to large temperature fluctuations, 
and therefore to rather large extremes of internal 
vacuum or pressure. Since the seals are not perfect, 
gas is alternately expelled and inhaled; in other 
words the instrument breathes. This, in the case of 
helium-filled instruments, causes contamination of 
the helium. In all instruments it leads to the intro- 
duction of a certain amount of water vapor, which, 
particularly in the case of Naval instruments or in 
the tropics, may cause the optics to fog. 

As soon as the use of helium had been standard- 
ized by the Services, steps were taken to determine 
how much the instruments would breathe. Results 
of this study were reported in a subsequent Report 
to the Services. (26) However, sufficient data was on 
hand to show that though instruments fresh from 
the factory frequently breathe very little, after they 
have been in service for a time many of them breathe 
a great deal, and occasional ones leak like sieves. 

This result had been expected and consideration 
had been given to possible methods of correction. 
The obvious possibilities are control of temperature 
and control of pressure. Of the two, temperature 
control seems the least desirable because it would 
require a constant source of power during periods 
of storage or transportation and, unless very care- 
fully worked out, might create more temperature 
problems than it eliminates. It was therefore decided 
to investigate methods of breathing by control of 
pressure. The American Gas Association Testing 
Laboratories developed such a device. Results of its 
use and the description of the apparatus are given 
below. In connection with these developments, it 
should be pointed out that if the pressure within the 
range finders can be successfully maintained within 
a very few ounces above atmospheric at all times, the 
seals of the instruments will in all probability last 
longer than at present, since it is probable that the 
initial breakdown of these seals is commonly caused 


by the extremes of pressure or vacuum due to tem- 
perature changes. 

Structural Changes 

This report then goes on to consider temperature 
effects on the structural parts of range finders. Me- 
chanical effects have been observed principally at 
low temperatures where bearings sometimes freeze 
and lubricants congeal. In the case of Army height 
finders Ml and M2, these effects are known to be 
serious. It is probable that a satisfactory solution 
would require redesign, but this would obviously 
not correct existing instruments now in the field. To 
alleviate the situation with respect to existing instru- 
ments, therefore, an electrically heated jacket was 
designed by the General Electric Company which in 
severe cold will keep the working parts of the instru- 
ment warm enough to assure freedom of movement. 
This was developed under the auspices of the Fire 
Control Design Section of the Ordnance Depart- 
ment, Frankford Arsenal, and is believed to be the 
most practical method of alleviation. 

Temperature effects in the structure of a height 
finder affect its optical performance in a number of 
ways. As the temperature of a lens rises, its radii of 
curvature increase, and its refractive index decreases. 
Both of these changes tend to increase focal length 
and lead to changes of adjustment of the perspective 
type. These will occur even if the temperature change 
is slow and uniform. When the temperature changes 
are nonuniform, more complicated distortions oc- 
cur. These appear not to have been adequately 
studied and are not well understood. Nonuniform 
temperature in mirrors, such as those used in penta- 
reflectors, causes their plane reflecting surfaces to 
become curved. This may either introduce lens 
action, thus leading to perspective errors, or may 
alter the angle of the beam relative to the axis. Non- 
uniform temperature in the supports of the penta- 
reflectors may cause an alteration of the angle be- 
tween the mirrors. This has been estimated and the 
magnitude has been found to be significant. 

Uniform temperature changes in the optical bar 
cause it to expand, thereby lengthening the distance 
between the various optical parts. That is advan- 
tageous, in some cases at least, since it tends to com- 
pensate for the increasing focal length of the lenses. 
Nonuniform temperature changes in the optical bar, 
however, cause curvature which, if in the plane of 


RESTRICTED 



42 


TEMPERATURE EFFECTS 


triangulation, may lead to range errors. Tempera- 
ture differences of the order of a degree between 
opposite sides of a bar are reported experimentally 
and it is estimated that a temperature difference 
of this magnitude would produce a 40-second curva- 
ture in the standard optical tube. 

The suggestion has frequently been made that the 
errors due to nonuniform expansion of these struc- 
tural parts could be largely alleviated by substituting 
invar and quartz in place of steel and glass. Some 
years before, the Bausch and Lomb Optical Com- 
pany constructed an invar optical bar. Keuffel and 
Esser had also experimented with bronze and with 
invar for the end-reflector supports, and with alumi- 
num for the optical bar. Officials of both companies 
state that no improvement was observed, probably 
because the effects under investigation were masked 
by errors due to stratification and other causes. At 
the time of this report an Ml and an M2 Army 
Height Finder were being equipped with invar opti- 
cal bars in order to permit an accurate comparative 
study to be made. An elaborate system of thermo- 
couples was also introduced into these instruments 
to measure internal temperatures accurately and 
continuously. The Ml instrument will have, in addi- 
tion, penta-reflectors consisting of quartz mirrors 
mounted in invar supports. 

It should be noted that many of the effects of non- 
uniform temperature, particularly those resulting 
from direct radiation, could be mitigated by the use 
of some form of sunshade. The practicability of such 
devices under field conditions is believed doubtful. 
One further effect of temperature changes in struc- 
tural parts is mentioned. If at any point the mechan- 
ical design of the instrument is such that expansion 
of a particular part can induce strains in sensitive 
optical supports such as the optical bar or the mounts 
for the penta-reflectors, errors may occur. Princeton 
Laboratory experiments revealed the largest tem- 
perature errors which have ever been observed. They 
were present in most but not all of the instruments 
tested, and their behavior was sufficiently erratic and 
mysterious that it was impossible to trace their ori- 
gin under the conditions of the experiment. Enough 
was learned, however, to indicate quite clearly that 
they were of mechanical origin, and their further 
study was transferred to the Eastman Kodak Com- 
pany, where instruments in various stages of assem- 
bly and subassembly were available for test. These 
tests have shown that the errors arose from pressure 


exerted on the optical bar by the change-of-magnifi- 
cation lever, and that they can be completely cor- 
rected by a very simple adjustment which can easily 
be made in the field. 

The above discussion has called attention to a 
wide variety of temperature errors. Attention is also 
called to suggested methods of alleviating most of 
these, either in instruments in the field, or by new 
design or construction of structural parts, or the use 
of new materials in the manufacture of these struc- 
tures. The process of correction so reported, how- 
ever, has been that of piling one modification after 
another on existing instruments. Such a process must 
ultimately lead to a situation where complete re- 
design is indicated. It was believed (October 1942) 
that this time had come; for the present instrument, 
when all the recommended gadgets are attached, has 
no unity of design. Moreover, the experience and 
technical information which has been gained in the 
course of these experiments is considerable, and 
would be of great value as a guide to redesign in a 
more fundamental sense. Much of it resides in the 
minds of the men who have been associated with the 
research program, and is unreported and intangible. 
Hence it is believed that a basic redesign of the 
stereoscopic range finder should be undertaken at 
that time. 

This last recommendation was put into effect and 
the results of this redesign program will be reported 
in a subsequent section of the present report. Later 
in this same section the results are summarized of 
experiments using new materials for some of the 
structural parts. 

Effect of Accuracy 

Attached to the Report to the Services just sum- 
marized above are five supporting documents giving 
in detail the theoretical and experimental basis for 
the statements made in that report. The first of these, 
on the effects of thermal instability on height finder 
accuracy is made by the Princeton Laboratory at 
Fort Monroe. (347) In this are given more complete 
experimental results and expanded theoretical dis- 
cussion of the effect of temperature stratification in a 
vertical plane on height finder readings. These data 
supply the quantitative values for the qualitative 
findings of the earlier Princeton report, with instru- 
ments charged with both nitrogen and helium and 
with thermocouple readings indicating the magni- 
tude of the temperature gradients. In some of these 


L RESTRICTF.n 


MISCELLANEOUS TEMPERATURE EXPERIMENTS 


43 


experiments a circulating pump was installed for 
the purpose of eliminating temperature stratifica- 
tion. Two instruments were kept at 0 degree C over- 
night. Upon removal from the cold room, both in- 
struments showed marked errors in range readings 
—the range sometimes short by 20 to 25 UOE. The 
maximum error was reached about an hour after 
removal from the lower temperature. It was 4 to 5 
hours before the instruments again read true range. 
The substitution of helium for nitrogen reduced the 
error to less than 3 to 5 UOE. The error remaining 
was too small to evaluate with great accuracy. The 
two instruments behaved differently possibly be- 
cause the aluminum temperature tubes and baffles 
were removed from one instrument and not from 
the other. The circulating pump was found unsatis- 
factory in its present design. 

A second Princeton Laboratory report describes 
a method of charging Height Finders Ml and M2 
with helium. (352) This is given in manual form and 
is to be used as a guide for methods of charging instru- 
ments in the field. A helium purity indicator is de- 
scribed and methods of determining the percentage 
helium in an instrument are given. Methods for charg- 
ing and testing height finders with helium without 
the aid of a helium purity indicator are also described. 

The third Princeton Laboratory report again 
deals with studies of range finder errors caused by 
temperature instability. (358) This report presents 
experimental evidence that serious errors exist in 
certain instruments when these instruments are 
subjected to too rapid changes of temperature or to 
nonuniform heating by the sun’s rays. In the Ml 
instrument these erroneous readings result from 
refractive errors caused by bending of light rays, and 
also from structural errors from bending of mechan- 
ical and optical parts. It is shown that the refractive 
errors are nearly eliminated through the use of 
helium as a charging gas. In this report the geometry 
of the Ml Height Finder is briefly and clearly given 
as a basis for the discussion. Errors as great as 11 
UOE are found for nitrogen-filled instruments under 
certain conditions. Experiments on mechanical 
effects were performed with an artificial sun consist- 
ing of a bank of 38 low-temperature 250-watt dry- 
ing bulbs as energy sources so that nonuniform 
heating effects could be obtained. When applied to 
the top of the height finder near the ends of the tube 
all but one of the instruments exhibited decreases 
in internal adjuster readings of 100 UOE or more 


in a few minutes after the artificial sun was turned 
on. These effects essentially disappeared if the instru- 
ment was depressed below about 60 degrees angular 
height. The effects were of about the same magni- 
tude independent of gas content. What range read- 
ings were available indicated that essentially all of 
the RCS change was caused by changes in the main 
optical path. Only very small effects were observed 
when the radiant heat was applied to the sides or 
above the central part of the instrument. Tests on 
two instruments showed that they, at least, had 
sensitive spots in the neighborhood of the internal 
adjuster lamp housings. For example, shielding these 
spots in one particular instrument reduced the ob- 
served RCS changes by a factor of 6. 

An Eastman Kodak Company report repeated 
these experiments with the artificial sun and found 
that one particular instrument exhibited a reduc- 
tion of 90 UOE in the internal adjuster reading with 
the finder set at 1,600 mils after the artificial sun 
had been on 30 minutes. (203) The second instru- 
ment showed only a small shift under the same con- 
ditions. It was found that when the clearance be- 
tween the change-of-power lever and the change-of- 
power disk was made adequate, the first instrument 
no longer exhibited the large shift in the reading 
but was like the second range finder. Examination of 
13 uninspected height finders revealed that the dis- 
turbing shift would take place in at least four. Of 
six instruments tested by the Princeton Laboratory, 
five showed the large effect. Hence it was recom- 
mended that in new range finders, the design of the 
change-of-power lever be altered to provide increased 
clearance between it and the change-of-power disk. 
Furthermore, for all M 1 Height Finders in the field, it 
was recommended that, as specified above, 1/16 inch 
washers be inserted between the bearing plate for the 
change-of-power lever and the boss on the outer tube. 

5 3 ELIMINATION OF THERMAL 
ERRORS 

A report from the National Bureau of Standards 
comments broadly on temperature errors and sug- 
gests a radical redesign to eliminate them. (320) It is 
pointed out that the chief causes for the inconsistent 
performance of range finders are: (1) temperature 
gradients in the air within the body of the instru- 
ment; (2) binding of the optical tube or bar, pro- 
duced either by mechanical strain or by temperature 


RESTRICTED 


44 


TEMPERATURE EFFECTS 


gradients; and (3) variation in the deviations pro- 
duced by the two penta-reflectors due to nonuniform 
expansion resulting from temperature gradients. In 
eliminating the errors due to bending of the optical 
bar a design is suggested for an optical tube with the 
two systems placed side by side instead of end to end. 
Fused silica end plates are also suggested. 

The Princeton Laboratory at Fort Monroe pre- 
pared a number of individual and more detailed 
studies which are summarized in the reports attached 
as confirming evidence to the Report to the Services 
discussed above. The first of these is concerned with 
RCS determinations on helium-filled and nitrogen- 
filled instruments. (439) It was found the RCS at 
zero elevation on the Ml instrument does not depend 
on the temperature level. Under conditions of tem- 
perature instability, RCS at zero elevation undergoes 
changes, probably connected with temperature dif- 
ferences within the instrument. These zero elevation 
changes are not primarily effects of refraction of the 
gas within the instrument, since they are unaffected 
by the substitution of helium for nitrogen. However, 
RCS at 1,600 mils elevation may differ from that at 
0 mils. This difference is affected by the gas within 
the instrument and may, therefore, be in part a re- 
fractive effect. 

The next report is a preliminary study of the 
effects of circulating gas within the height finder on 
internal temperatures. (440) Another report, a theo- 
retical study, concerns the effects of temperature 
stratification on height finder accuracy. (441) The 
technique of testing and flushing helium-charged 
height finders is the subject of the next report. (442) 
This gives detailed procedures already referred to 
above. 

A report by the Princeton Laboratory deals with 
experiments to determine quantitative relations be- 
tween thermal and optical conditions in an M2 
Height Finder. (443) It describes in some detail the 
experiments carried on by cooling the instrument 
overnight in the cold chamber. The experimental 
results obtained are given in a following report. 
(444) From this work it is concluded that at 1,600 
mils elevation the averaging thermocouples were 
highly successful in predicting the refractive effects 
in the main optical path. Also, the use of helium as 
a charging gas reduced the temperature gradients 
developed under these conditions by a factor of 5. 

Another report from the Princeton Laboratory is 
concerned with the kinetics of the warming process 


in a modified M2 Height Finder. (445) This consists 
of a further analysis of results obtained from keep- 
ing the instrument in a cold room overnight. It was 
found that the relaxation time with no helium is 
1.5 to 1.7 times as large as that with pure helium. 
Thus, in the helium-filled instrument much of the 
heat flow (30 to 35 per cent) is carried off by the 
helium. It is believed that the warming of the optical 
bar is the controlling factor in this warming process. 
It was found that a purity of 54 per cent helium was 
little better than no helium in cutting down the 
relaxation time. Another report describes in detail 
the theory and apparatus for the experiments with 
the artificial sun. 

It was pointed out above that range finders breathe 
and that, unless precautions are taken, replenish- 
ment would have to be of fairly frequent occurrence 
in order to maintain adequate helium purity. It was 
also suggested above (16) that one way of dealing 
with this problem would be to maintain a gas pres- 
sure within the range finder slightly above the pres- 
sure of the outside air. The Fire Control Division of 
NDRC has prepared a Report to the Services on this 
problem of the retention of helium in range and 
height finders (26). All range finders breathe more or 
less. That is, as the temperature rises, the internal 
pressure increases and gas escapes through imper- 
fections in the seals. As the temperature falls, the 
internal pressure also falls, and when it becomes 
lower than the atmospheric pressure, air is sucked in. 
This breathing is undesirable under any circum- 
stances, since it introduces moisture into the instru- 
ment. It is particularly undesirable in the case of 
helium-charged instruments because the purity, and 
therefore the effectiveness, of the helium charge is 
reduced. As soon as it became evident that stratifi- 
cation errors could not be effectively reduced in 
American range finders by pump agitation, and that 
the use of helium was necessary, a study was under- 
taken to determine how rapidly the helium became 
contaminated. This study was begun at Fort Monroe 
on several instruments that were being used in the 
Height Finder School, and was extended to include 
the performance of Army height finders and Navy 
range finders under Service conditions as soon as 
the adoption of helium by the Services made this 
possible. 

The detailed report from the Princeton Labora- 
tory is attached to this report to the Services. (369) 
Stated briefly, the study showed that the majority of 


RESTRICTED^ 


TECHNIQUES OF HELIUM CHARGING 


45 


instruments were tight enough to hold helium for 
periods ranging from 1 to 6 months. It was believed 
that most of the remainder could be made reasonably 
tight by repairing the gland packing and other seals. 
Helium purity indicators were used and internal 
adjuster readings were taken on at least one helium 
charge of 445 Army and Navy instruments. The 
helium purity indicator proved a satisfactory testing 
instrument. To insure that the range finders be kept 
at an average helium percentage of 90 per cent, with 
sufficiently small variation to give satisfactory service, 
recommendations were made that all range finders 
which are not excessively leaky be charged, and when 
necessary recharged, to 95 per cent helium, and that 
any range finder which tests below 88 per cent be 
regarded as requiring recharging. In an appendix to 
this Princeton Laboratory report the Oliver Helium 
Charging Method is given in detail. Detailed data 
regarding leakage is given in another appendix. 


5.3.1 Prevention of Helium Contamination 

While these studies (26) of helium retention were 
under way, the American Gas Association Testing 
Laboratory was asked to develop means for control- 
ling the breathing process, thereby preventing heli- 
um contamination. They developed an attachment 
for this purpose which is described in four reports 
appended to the Report to the Services. (62, 63, 64, 
65) Briefly, it consists of a bellows directly connected 
to the interior of the height finder permitting the gas 
to expand and contract freely at a pressure only 
about 1 ounce above atmospheric; a relief valve 
which allows gas to escape when the bellows is fully 
expanded, thereby preventing high internal pressures 
from developing; and a small high pressure helium 
supply from which helium is introduced into the 
range finder through a sensitive reducing valve when 
necessary to prevent the internal pressure from fall- 
ing below atmospheric. The apparatus has been so 
engineered that it can be attached to existing range 
and height finders by merely replacing the desic- 
cating plug by a suitable adapter. In the case of the 
Ml Army height finder, it has been designed within 
the space limits of the present carrying case, so that 
the breathing apparatus will continue to function, 
and contamination of helium will continue to be 
prevented even during the periods of storage or trans- 


portation. The apparatus was tested under a variety 
of conditions. 

Strictly speaking, the only portions of this appara- 
tus which are necessary to prevent helium contami- 
nation are the supply tank and sensitive reducing 
valve. It is therefore desirable to explain why the bel- 
lows and relief valve were added. This can best be 
done by assuming that we had, initially, a very tight 
instrument to which were attached only the supply 
tank and reduction valve. In such an instrument the 
pressure could never go below atmospheric. How- 
ever, as the temperature rose, a very considerable in- 
ternal pressure would develop. At the same time, the 
seals would be softened by heat and under extreme 
conditions would break down and allow gas to es- 
cape. Once being weakened by such a breakdown, 
they would fail easily on subsequent occasions. 
Therefore a situation might be expected to develop 
in which substantially all of the gas introduced 
through a period of falling temperature would leak 
out during the subsequent rising part of the cycle, 
thus leading to an excessive consumption of helium. 

That this process actually does take place is con- 
firmed by a test reported by the American Gas Asso- 
ciation Testing Laboratories. (64) In this test, an 
Army height finder, originally in better than average 
condition, was subjected to successive cycles of rising 
and falling temperature under carefully controlled 
laboratory conditions. During the first cycle of rising 
temperature, the internal pressure rose in normal 
fashion to a maximum value of 26 inches of water, 
at which point it abruptly fell to about 4 inches, 
though the internal temperature at the time was 
still rising. The damage to the seals in this cycle was 
sufficiently great that the internal pressure thereafter 
never rose above 6 inches of water. The bellows and 
relief valve have been included in the A.G.A. equip- 
ment to prevent such occurrences. They reduce the 
consumption of helium both by keeping the instru- 
ment tight and by keeping the pressure differential 
so low that only a small amount of gas escapes even 
from a moderately leaky instrument. 

Another possible method of discouraging breath- 
ing is by maintaining the entire instrument constant- 
ly at a temperature above the maximum to be ex- 
pected from diurnal variations. This could be done, 
for example, by means of an electrically heated 
jacket. There was no background of experience upon 
which to base an appraisal of its merits, but three 
disadvantages may be suspected on theoretical 


46 


TEMPERATURE EFFECTS 


grounds: (1) the difficulty of maintaining the system 
in operation during periods of transportation or 
storage; (2) contamination due to breathing induced 
by changes in barometric pressure (the range of 
which is about 10 per cent of mean atmospheric 
pressure and therefore equivalent to a temperature 
range of about 40 F); and (3) optical disturbances 
induced by inequality of heat transfer. The method 
was therefore not regarded as desirable. 

The Fire Control Division of NDRC recom- 
mended that the A.G.A. pressure control attachment 
be adopted for all helium-filled range and height 
finders. The recommendation by the Princeton 
Laboratory in regard to the Oliver helium charging 
method is not endorsed. It is pointed out that the 
Oliver method will guarantee adequate helium 
purity under only one specific set of circumstances, 
namely, when an instrument is being recharged. It 
is therefore a valuable method for use when a helium 
purity indicator is not available. It does not, how- 
ever, give an indication of the percentage contami- 
nation of the helium. Such an indication is desirable, 
both in order to know when recharging is necessary, 
and in order to detect and correct promptly any 
unusual causes of contamination. Hence, a satisfac- 
tory helium purity indicator is believed preferable 
to reliance on the Oliver method. Such a purity indi- 
cator was subsequently developed by the American 
Gas Association Testing Laboratories under the 
auspices of the Navy Bureau of Ordnance. 

There are several more detailed studies by the 
Princeton Laboratory at Fort Monroe on matters of 
temperature control in range finders. One deals with 
the retentivity of range finders and height finders 
in the field. (448) A second study describes the Oliver 
helium charging method. (450) A third outlines cer- 
tain tests made at the Eastman Kodak Company 
using the artificial sun and reports, for one instru- 
ment, range errors as great as 30 UOE when the 
instrument was so heated. (451) These errors were 
reduced after the instrument was flushed with helium 
or was partially evacuated. 

A Princeton Branch Frankford Arsenal report 
deals with oil contamination from ordinary helium. 
(236) It discusses the question of whether the use of 
ordinary helium as a charge for height finders would 
result in serious contamination of the optical sur- 
faces by oil. After outlining such matters as the 
amount of oil required for serious contamination, 
the expected amount of oil in ordinary helium and 
the like, the conclusion is reached that serious con- 


tamination is not to be expected. 

Certain comments and experiments on tempera- 
ture control have been reported by the British. ARL 
reports an investigation of the extent to which an 
excess internal pressure can be retained in a modified 
American Ml instrument. (102) At the commence- 
ment of this test, the range finder was charged with 
air at a pressure of 4.5 psi and the subsequent decrease 
in pressure, observed over a period of 30 days, was 
noted. The instrument was used on the average of 
2 to 3 hours per day. It was found that the pressure 
decreased to about 1 psi in the course of the first 
week. Subsequently the decrease was slower and 
even after 30 days there still remained an excess pres- 
sure of 0.25 psi. The degree of sealing efficiency re- 
vealed by this test is unapproachable by that of any 
British instrument examined up to that time by 
ARL. With British instruments, excess of internal 
pressures are held for a matter only of seconds or 
minutes. The report indicates reduction of elevation 
errors by air stirring under Service conditions. It is 
perhaps because of this inadequate sealing of the 
British instruments, that the British resorted to air 
stirring rather than the use of helium to reduce ele- 
vation errors. Indeed, in another British report, this 
is rather explicitly stated. (105) 

Another British report describes experiments deal- 
ing with elevation errors in a U. S. Ml Height Finder 
at ARL. (103) With air filling, the elevation errors 
were found to be rather less than with an 18-foot 
British instrument. Type FQ 25, fitted with internal 
temperature tubes. Filling the range finder with 
hydrogen in place of air has been found to reduce 
elevation errors to approximately half the values 
obtained with the air filling, while, with helium 
filling, elevation errors were reduced to about one- 
third or one-quarter. The reduction of elevation 
error effected by charging the instrument with heli- 
um is substantial but, on occasion, errors of —2 to 
—3 seconds in the object space were detected. The 
zero elevation temperature coefficient was found to 
be approximately 0.14 seconds per degree centigrade 
and therefore similar in magnitude to those present 
in the average British range finder. 

Air Stirring 

Still another British report by ARL deals with a 
study of the removal of elevation errors by circum- 
ferential air stirring in the U. S. Ml Height Finder. 
(50) In this instrument, the internal beam shrouding 


RESTRICTEl 


COLD WEATHER TESTS 


47 


tubes were removed and air jet tubes were filled to 
permit the application of circumferential air stir- 
ring. Tests were then carried out to determine the 
efficiency of air stirring as a means of removing ele- 
vation errors in the instruments. The results show 
that air stirring, either by means of a hand pump or 
by means of a motor-driven pump (the Naval pat- 
tern Q.G.2) reduced elevation errors of all magni- 
tudes to residual errors of the order of 1 second. A 
comparison of the results obtained with air stirring 
with those obtained with helium filling, in the Brit- 
ish study reported above, shows that air stirring ap- 
pears to be slightly more efficient than helium filling 
in reducing elevation errors. With the internal beam 
shrouding tubes removed, it was found that, as ex- 
pected, the internal adjuster did not give an accurate 
means of checking the errors of the Height Finder 
unless air stirring was in operation. Comment may 
be made regarding the final conclusion reached by 
this report that air stirring is slightly more efficient 
than helium charging. The air stirring of this report 
is compared with results of the previous ARL report 
on helium charging. (103) In regard to the helium 
study, for some reason the introduction of helium 
failed to produce as great a reduction in elevation 
error as should theoretically occur. The causes of 
this failure were not discovered. In view of the fact 
that the American experiments have repeatedly se- 
cured the theoretical measure of improvement, it is 
natural to suspect that a part, at least, of the elevation 
error observed in the British experiments was due 
to causes other than stratification, perhaps to some 
form of mechanical strain causing flexure in the 
optical bar. It is these data, which are not as favor- 
able as would be expected, which are used as the 
basis of comparison with air stirring. 

In an effort to reduce stratification due to tempera- 
ture changes by partially shielding the range finder 
from the radiant heat of the sun, and hence supple- 
ment the helium charging of the instrument. Brown 
University reports the development of a height finder 
sunshade. (152) This can be readily assembled in 
the field from wood and canvas and is easy to install 
or remove. It covers the two ends of the instrument 
beyond the tracking telescopes. Detailed description 
and procedure for assembly are given. 

Cold Weather Tests 

A series of cold weather tests of height finder per- 
formance is reported by the Frankford Arsenal. (251) 


The tests were designed to determine the effects of 
M404 and M405 electrically heated covers on the 
mechanical and optical performance of the height 
finders under actual Arctic field conditions. The 
tests were carried on under conditions of extreme 
cold in north-central Canada during 10 weeks from 
January to March. Two Ml and two M2 instruments 
were tested continually throughout this period, both 
with and without the use of the two electrically heat- 
ed covers. During this period, the temperature varied 
from —34 to -\-l6 F, with a mean temperature of 
—10 F. Thermal stratification, as determined by 
both thermocouple measurements and internal ad- 
juster readings at various elevations, was found to 
be negligible without cover heat and considerable 
when heat was applied. In most cases, the tempera- 
ture gradient pattern reached stability quickly, about 
2 hours after the application of heat, and appeared 
to be independent of the internal temperature which 
reaches stability relatively slowly after about 20 
hours. The internal temperature gradients may be 
explained by appreciable external gradients pro- 
duced by the heated covers and are progressively re- 
duced at ambient temperatures of —5 F and higher. 

Aerial targets of known position were not avail- 
able for testing purposes. The Ml Height Finders 
functioned satisfactorily in ranging to ground tar- 
gets under all operating conditions, but the M2 
instruments functioned satisfactorily only when 
cover heat was applied. The Ml Height Finders 
tested were, with few exceptions, superior in mechan- 
ical operation to the M2 Height Finders at all tem- 
peratures. In fact, without the installation of the 
covers the Ml instruments operated satisfactorily at 
temperatures as low as —30 F and functioned in a 
normal manner at +16 F. The installation of the 
cover and the application of heat improved the 
mechanical operation in general. With heat, the Ml 
instrument operated in a normal manner at —20 F. 
However, some of the M2 mechanisms operated un- 
satisfactorily at —13 F, even with the heat. 

It was observed that ocular fogging is probably the 
most serious difficulty encountered in the use of the 
height finder at sub-zero temperatures. This difficulty 
may be offset by the use of facial masks, of blankets 
placed over the observer’s head and the instrument, 
or of heating coils to raise the temperature of the 
eye lens of the ocular. Helium charging, using either 
the helium purity indicator or the Oliver technique, 
presented no major difficulties at sub-zero tempera- 
tures. It was found that rapid temperature drops 


^^RE.SfRICT g^ 


48 


TEMPERATURE EFFECTS 


increase the rate of helium leakage in the height 
finders, whether heated covers were used or not. 
Examination of the optical elements and various 
standard optical checks indicated that sub-zero tem- 
peratures (down to — 34F) did not produce any seri- 
ous optical difficulties nor were electrical defects of im- 
portance encountered in either type of instrument. 

On the basis of these results it was recommended 
that Ml Height Finders be used without the electric- 
ally heated covers whenever their mechanisms will 
function properly in this condition and that M2 
Height Finders should not be used for Arctic opera- 
tions. If the cover becomes necessary for the Ml, heat 
should be applied at least 10 hours prior to use. 

A Fort Monroe Princeton Laboratory report out- 
lines experiments which show that the calibration of 
the height hnder is materially altered when helium 
is used as the charging gas. (449) In six different 
experiments, an instrument was set up and left me- 
chanically undisturbed throughout the sequence of 
observations. Two or more observers read ranges on 
a fixed target and after several such readings, the gas 
content of the instrument was changed. The readings 
were then repeated with the new gas content. In one 
experiment the mean change from helium to nitro- 
gen filling was —65 yards or about —4 UOE. In the 
final experiments in spite of observer differences the 
change from helium to nitrogen resulted in differ- 
ences in range readings from 13.0 to 2.5 UOE and 
in the RCS setting from -[-4.9 to —2.7 UOE. 

A report from the Naval Inspector of Ordnance at 
the Bausch and Lomb Optical Company reports 
experiments on the determination of parallax result- 
ing from the charging with helium. (318) It was 
found that range accuracy is affected by the intro- 
duction of helium into the range finder, the magni- 
tude of the errors depending upon the original ad- 
justment of the instrument in air. With proper inter- 
pupillary setting, the average range error when the 
instrument is filled with helium is less than 1 UOE. 
The range errors with improper interpupillary set- 
tings in air and in helium are of such magnitude as 
to obscure the actual effects of helium. The average 
range errors due to the introduction of helium are 
not greater than errors existing between various 
range finders due to inherent optical or mechanical 
adjustments. However, it was found that a change 
of focus due to the introduction of helium was about 
0.2 diopters, which closely approximates the calcu- 
lated distance. 


This distance was calculated by the Princeton 
Laboratory at Fort Monroe for the Ml Height 
Finder and was found to be very small. (517) An- 
other Fort Monroe report details another calculation 
of the change of focus in the M 1 Height Finder which 
may be expected when changing from nitrogen to 
helium. (516) The change in focus of the objectives 
is calculated as 0.411 mm corresponding to a shift 
of image from infinity to 568 yards. The internal 
adjuster objectives shift 0.93 mm corresponding to a 
shift from infinity to 1,340 yards. The image of the 
internal adjuster thus shifts relative to the reticle 
an amount corresponding to a shift from infinity to 
400 yards. 

A Frankford Arsenal Princeton Branch memor- 
andum describes an instrument, called the mono- 
focle, which is designed for use with the Ml Height 
Finder to measure the parallax between target and 
reticle. (232) This instrument indicates when the 
reciprocal range of an observed image differs from 
zero by more than a certain amount, expected to be 
approximately 0. 1 diopter, or when the range to the 
image is less than a certain amount, expected to be 
about 10 meters. The theory and preliminary design 
of the instrument are given in the report. 


5.3.4 Measurement of Focal Differences 

A Frankford Arsenal Princeton Branch report 
studies the measurement of focus difference in the 
Ml Height Finder. (263) Irregular aberrations of 
substantial size have been observed in the target 
images formed by an Ml Height Finder. Mathemati- 
cal study shows that, in the presence of such aberra- 
tions, the focus determined by a dioptometer will not 
give accurate indication of the focus difference be- 
tween target and reticle effective in creating perspec- 
tive error. The study describes an experimental model 
of a simple device, the aperture oscillator, which was 
designed and built. This instrument measures the 
focus differences effective in causing perspective 
error. It is as precise, under the conditions tested, 
as a dioptometer. The recommendation is made that 
aperture oscillators be used for checking height 
finder focus in the field and in inspection of new 
instruments and that consideration be given to the 
use of a similar instrument whenever lateral parallax 
must be avoided. 


RESTRIC 


EXPANSION OF OPTICAL BARS AND PENTA-REFLECTORS 


49 


5 4 THERMAL EFFECTS ON OPTICAL 
BARS AND PENTA-REFLECTORS 

In a Report to the Services summarized above, 
(16) it was pointed out that some of the thermal 
effects in range finders might be alleviated by the 
introduction of certain primary structures which 
would be less affected by temperature changes and 
thermal gradients. The results of several attacks on 
these problems will be described below. 


Comparison of Invar and Steel 
Optical Bars 

The Bausch and Lomb Optical Company reported 
laboratory tests on the comparative performance of 
invar and steel height finder optical bars. (114) Be- 
cause field experience showed a strong probability 
that temperature effects on height finder optical bars 
introduced important ranging errors, a sample M2 
bar was made of invar steel. Laboratory tests were 
made on this bar, on a standard steel bar, and on a 
standard steel bar with a polished nickel plate ap- 
plied to the outside surface. These tests included: 
(1) the determination of the coefficient of linear ex- 
pansion of the metals, using a one metal gauge length 
of the optical bars; (2) a determination of bending 
curves under induced transverse temperature gradi- 
ents; (3) a search for parallax and other optical 
effects resulting from uniform temperature changes 
in the assembled optical bar; and (4) observations of 
range errors induced by transverse temperature 
gradients in the plane of triangulation. The experi- 
mental arrangements are described in the text. It 
was found that the coefficient of linear expansion, 
per degree C, within temperature ranges of 0 to 100 C 
averaged 1.8 for invar as against 11.55 for steel. The 
average radii of curvature under transverse tempera- 
ture difference of 1 C were 0.99 for invar, 1.80 for 
standard steel, and 1.62 for bright nickel-plated steel. 
In the uniform temperature tests, over a range from 
8 to 39 C with normal interocular separation, the 
range readings were erratic. Precision of an indi- 
vidual set of readings was less than 1 UOE. Consist- 
ency between sets was about 5 UOE. Changes in 
median of sets from one temperature to another were 
not over 5 UOE. No definite conclusions for this 
variability are drawn, but support friction may ex- 
plain the fluctuations. Changing the interocular dis- 


tance by ±2 mm from the observer’s interpupillary 
distance indicated parallax errors of about 3.5 UOE 
for the steel bars, and none for the invar bar. In re- 
gard to the range errors introduced by transverse 
temperature gradients, measured in UOE per degree 
C, the experimental results were very close to the 
theoretical calculated values of 2.3 for invar, 12.4 
for standard steel, and 13.8 for nickel-plated steel. 
Hence it was determined that the materials have 
approximately their expected characteristics when 
fabricated as optical bars, and that the mechanical 
effects of transverse temperature gradients are propor- 
tional to the coefficients of linear expansion. As noted 
above there is indication of an uncontrolled factor 
in these experiments whose effect is not negligible. 
There appeared to be an erratic thermal lag, which 
may have been due to a combination of differential 
expansion between the bar and the temperature 
control jacket with friction in the supports. A reason- 
able stick-slip friction force may readily have intro- 
duced erratic errors of the kind found. Because of 
the nature of the experimental heating and cooling 
processes, differential expansion of the same order 
of magnitude was to be expected with the steel bar 
as with the invar bar. However, the results indicate 
that, in spite of these erratic fluctuations, the use of 
invar is capable of reducing range errors due to 
transverse temperature differences in the plane of 
triangulation by a factor of at least 5 to 1. Since 
field experience has indicated temperature differ- 
ences across the bar sufficient to introduce ranging 
errors of approximately 20 UOE, it is obvious that a 
change to invar would be well worth while in height 
finder applications. 

The investigators conclude from these data, for 
applications similar to height finder service where 
relatively large temperature gradients in the plane 
of triangulation may be expected, a design with an 
invar optical bar is to be recommended. For applica- 
tions in which the instrument will be so shielded 
from solar radiation and the like as to minimize the 
danger of setting up large temperature gradients, 
invar cannot be unequivocally recommended. Means 
must first be found to reduce erratic errors arising 
from uniform temperature changes. If further ex- 
periments indicate that support friction is the chief 
cause of erratic behavior, invar optical bars may be 
recommended subject to improvement of the sup- 
port mechanism. 

A second report from Bausch Sc Lomb gives results 


restricte: 


50 


TEMPERATURE EFFECTS 


of additional laboratory tests of comparative per- 
formance of invar and steel optical bars. (116) These 
experiments indicate that the erratic errors arising 
from uniform temperature changes in the optical 
bar, found in the previous experiment, have support 
friction as a primary cause. This source of error was 
eliminated either by: (1) applying mechanical shock 
to the jacket enclosing the optical bar, when using 
the regular support, before recording range observa- 
tions; or (2) supporting one end of the optical bar 
with a bifilar piano wire suspension. From these 
studies the investigators conclude: 

1. For applications similar to height finder service, 
where relatively large temperature gradients in the 
plane of triangulation may be expected, a design 
with an invar optical bar is to be recommended. It 
must be recognized, however, that such a change does 
not account for other factors in range finder con- 
struction which have been eliminated from these 
experiments and which may tend to produce large 
errors under the above conditions. 

2. For applications in which the instrument wdll 
be so shielded from solar radiation as to minimize 
the setting up of large temperature gradients in the 
optical bar, there is no appreciable difference in per- 
formance between invar and steel. The choice of the 
material may be safely dictated by other considera- 
tions. 

Penta-Reflector Errors 

A report from the Eastman Kodak Company 
deals with the effect of temperature gradient 
on mirror flatness. (202) There is first a theo- 
retical discussion of the effects of different tem- 
peratures in the front and back surfaces of a plane 
mirror. The reflecting surface would then assume a 
spherical shape, concave if the temperature of the 
rear surface was greater than the front and convex 
if the temperature differences were in the other di- 
rection. With the present glass mirrors, using a height 
finder at 24 power, a focus shift of 0.007 seconds at 
the main reticle represents a 0.014-second shift at the 
eyepiece. The eyepiece focal length is about 28 mm 
so that a focus shift of 0.032 at the eyepiece repre- 
sents 1 diopter. Hence a temperature difference of 
1 F between the front and rear surfaces of one end 
mirror will cause parallax equivalent to 0.5 diopter 
measured at the eyepiece at 24x magnification. The 


effect varies directly with the temperature difference. 
If both end mirrors at one end of the height finder 
have equal temperature difference between their 
front and rear surfaces, the resulting focus shift is, 
of course, twice that which would result from the 
same temperature difference between the surfaces of 
just one mirror. When the mirror is made of quartz, 
which has a coefficient of linear expansion smaller 
than that of glass, the deformation is about one- 
twentieth as great, and the focus shift would be only 
one-fortieth diopter per degree Fahrenheit. To test 
these calculations, an experiment was performed 
using height finder mirrors of crown glass and of 
quartz. The curvature of the mirror was measured 
in terms of interference rings on a Twyman-Green 
interferometer. With both crown glass and quartz 
the experimental results closely approximated the 
calculated values. For the mirror of quartz, the path 
difference is only 10-5 mm per degree F, or in rings, 
one ring of interference for every 40 F temperature 
difference, while for crown glass it was approximately 
one ring for every 2 F temperature difference. 

A further experiment was carried on by the East- 
man Kodak Company in regard to the effect of tem- 
perature gradient on distortions of thin slabs. (207) 
Temperature gradients can be set up in penta-reflec- 
tors in any one of four ways: (1) radiation from 
nearby structures; (2) convection currents within 
the tube; (3) conduction by way of the supporting 
casting; and (4) temperature changes due to tem- 
perature gradients in all parts of the instrument. 
Each of these possible factors is considered theo- 
retically in the present study. The conclusion is that 
distortion of the penta-reflectors does not depend 
on the thermal expansion coefficient of the material 
but also on the heat conductivity of the mate- 
rial. Curves of the curvature of slabs of different 
thickness made of various materials (glass, steel, 
quartz, invar, aluminum, copper, invar and alumi- 
num, and quartz and copper) are given and the last 
named combination gives the best results. As a result 
of these findings, a composite block of quartz and 
copper was constructed and subjected to preliminary 
tests. These tests indicated that such composite 
blocks are more than sufficient to eliminate deviation 
errors in penta-reflectors. 

Another Eastman Kodak Company report deals 
with the effect of temperature gradients in range 
finder penta-reflectors and carries still further the 
preliminary experiments noted just above. (205) It 


RESTRlcfilT^ 


QUARTZ-COPPER PENTA-REFLECTOR ASSEMBLIES 


51 


is pointed out that the conventional types of range 
finder penta-reflectors consist of two plane mirrors 
securely mounted to a common support which main- 
tains a fixed angular relation between them. From 
a functional standpoint it is important that there be 
no significant change in this angular relation, par- 
ticularly in the plane of triangulation, over periods 
of time during which it is not feasible to make some 
compensation for the error. Previous investigations 
supported by data gathered in conjunction with this 
study have shown the existence of rather large tem- 
perature gradients in range finders subjected to con- 
ditions of varying ambient temperature. It was the 
purpose of the present experiment to study the effect 
of these temperature gradients on the light devia- 
tions of six types of penta-reflectors. The experiment 
was deliberately controlled to preclude the presence 
of significant gradients across the mirrors themselves. 
Hence these investigations are concerned solely with 
errors resulting from deformation of the mirror sup- 
port due to the presence of temperature gradients. 
Measurements were made with an interferometer. 
Temperature gradients were measured by thermo- 
couples. Standard penta-reflectors from the Ml and 
M2 Height Finders, the Mark 58 Range Finder, a 
quartz assembly obtained from the French battle- 
ship Richelieu, and a quartz assembly furnished by 
the National Bureau of Standards were tested and, 
in one experiment, also a Zeiss all-quartz block. 

A first experiment to produce a maximum effect 
in the triangulation plane and for this purpose the 
heat source was placed directly behind the block. 
A suitable mask prevented radiation from falling on 
any but the rear surface area of the block. As the 
block was heated, simultaneous readings of the tem- 
perature gradient between front and rear surfaces 
and the orientation and number of interference 
fringes in the interferometer field were made at fixed 
intervals of time until the block reached a steady 
state of temperature gradient. At this point the heat 
was turned off and readings recorded until the block 
was once again in a stable condition. In a second 
experiment, to produce a maximum effect at right 
angles to the triangulation plane, the heat source 
was placed directly above the block. 

The investigators conclude from these findings 
that the Ml block is obviously the poorest type, 
showing very significant deviation changes in both 
experiments. When allowance is made for the con- 
vection losses in the quartz blocks, the Richelieu 


assembly in particular is seen to behave quite poorly 
under the conditions of the first experiment. The M2 
block seems very good, certainly the best in the first 
experiment and comparing favorably with the best 
in the second experiment. The Mark 58 shows up 
about average. All the blocks tested showed signifi- 
cant changes, i.e., changes which might well affect 
the accuracy of a range finder were they not com- 
pensated for in some manner. 

From these observations it appears that the tem- 
perature effects in these blocks are not alone depend- 
ent on the coefficient of linear expansion of the mate- 
rial of which it is made, but also on the heat con- 
ductivity and the geometry of the block. For example, 
in the Ml the top and bottom portions are very thin 
and connected by members of a very small cross sec- 
tion, while the Mark 58 has relatively thick top and 
bottom portions and large connecting members. 
This fact, coupled with its smaller over-all size un- 
doubtedly explains the better behavior of the Mark 
58 block in both experiments. Simularly, the appar- 
ent superiority of the M2 block over the Richelieu 
seems to arise from the relatively large conductivity 
of the steel over the quartz, coupled with its smaller 
thickness. These two factors seem to more than 
cancel the effect of the very low coefficient of expan- 
sion of quartz. Hence, apart from the very necessary 
consideration of geometry, it appears that the best 
block is one with the highest ratio between its heat 
conductivity and its coefficient of linear expansion. 
On the basis of this criterion the present types of 
penta-reflector blocks are not satisfactory. 

The Eastman Kodak Company designed several 
penta-reflectors in accordance with these principles 
and reports of their testing are contained in another 
report. (209) They are of composite construction 
and were found to be many times better than neces- 
sary for adequate range finder performance. The 
results of previous experiments indicate that quartz 
mirrors are probably preferable to glass for most 
installations, and that a quartz-copper or quartz- 
aluminum composite block approaches the ideal type 
of penta-reflector system more closely than the other 
designs which were tested. The results of these ex- 
periments seem to support the theoretical arguments 
pertaining to proper penta-reflector design: (1) that 
the angle determining member should be of some 
material with a very low linear coefficient of expan- 
sion, in which the presence of small temperature 
gradients will be insignificant; (2) that this member 


RESTRICT] 



52 


TEMPERATURE EFFECTS 


be completely surrounded by a material of very high 
heat conductivity so as to prevent the formation of 
temperature gradients; and (3) that the outer shield 
be so mounted that it cannot transmit mechanical 
stresses to the inner member. 

The copper-quartz composite block was designed 
primarily to fulfill these conditions. In its construc- 
tion, two quartz mirrors are cemented to a quartz 
spacer. The spacer of this unit is then completely 
surrounded by a thick copper shield— the mirrors 
protruding up through this shield, which is separated 
from the mirrors and spacer at all points by an air 
gap of approximately 1 mm. Both the spacer and 
the inside of the copper shield were painted dull 
black. In the experimental design, the quartz was 
supported inside the copper shield between two 
small metal spring rings, although it is recognized 
that this is not a practical production design. 

This composite copper-quartz assembly was tested 
comparatively against the standard Ml and M2 steel- 
mounted assemblies, an Ml and M2 block mounted 
on invar steel and a composite aluminum-steel as- 
sembly. It proved that the copper-quartz assembly 
was better than the standard Ml, for top heating by 
a ratio of 45 to 1, and for back heating by a ratio of 
24 to 1, combining the results both for heating and 
relaxation. The maximum deflection for the Ml is 
almost 23 seconds as against a maximum deflection 
for the copper-quartz block of 0.5 second for back 
heating. For top heating these values are, for Ml, 
28.6 seconds, and for the copper-quartz, less than 
1.5 seconds of deflection. 

The Eastman Kodak Company have reported fur- 
ther work on the reduction of range finder errors by 
modification of the penta-reflectors and the mount- 
ing of the optical bar. (210) This was a highly prac- 
tical study to determine what modifications could be 
incorporated into the present Ml Height Finders to 
minimize these two sources of error and to determine 
methods by which these errors can be eliminated in 
an instrument of new design. To this end the charac- 
teristics of the range errors in the Ml Height Finder 
were investigated. One error reaching a maximum of 
8 to 10 UOE between 1 and 2 hours after subjecting 
the instrument to a temperature change is caused by 
thermal gradients in the penta-reflectors. A second 
error often reaching a maximum of 30 to 40 UOE 8 
to 10 hours after the temperature change results 
from mechanical stresses in the optical bar. 

The present investigation has shown that heat is 


carried to and from the penta-reflector block pre- 
dominantly by conduction through the supporting 
casting. Large temperature gradients are set up in 
the penta-reflector block as the heat is conducted to 
one side of the block which, in turn, is so constructed 
that the heat conduction across it is very small. These 
temperature gradients produce distortions in the pen- 
ta-reflector block which change its reflecting angle. 

Three possible methods of reducing the tempera- 
ture gradients in the penta-reflector were investi- 
gated: (1) supporting the block on thermal insula- 
tors; (2) supporting the block from both top and 
bottom by steel diaphragms; and (3) covering the 
block with flexible copper braid. It was found that 
mounting the penta-reflector block on thermal insu- 
lators was undesirable because of temperature gradi- 
ents resulting from convection currents. These gradi- 
ents were about one-third as large as those set up by 
conduction. Supporting the block both top and bot- 
tom proved not to be feasible. The gradients between 
the top and the bottom of the block were greatly 
reduced in this case, but there was a large tempera- 
ture difference between the front and the back of the 
block where the effect on the reflecting angle is a 
maximum. It was found that covering the penta- 
reflector block with flexible copper braid fastened to 
the top, bottom, and back of the block provided the 
most satisfactory solution. When enough copper 
braid was added to fill the available space surround- 
ing the penta-reflector block, the temperature gradi- 
ents were reduced to about one-third those in the 
unshielded block. The advantages of using the pres- 
ent supporting casting and mount offset any benefits 
to be derived from reducing the gradients further 
by the use of more shielding. This leads the investi- 
gators to outline the design of an ideal penta-reflec- 
tor. It is obtained by designing to provide good heat 
conduction between all the parts so that temperature 
gradients will be impossible. This can be accom- 
plished in two ways: (1) by making the separator out 
of a good conductor, such as copper, or (2) by making 
it of a material with low thermal expansion, as quartz 
or invar, which in turn is surrounded by a good con- 
ductor such as copper or aluminum. Some provision 
must be made for conducting the heat over the en- 
tire surface between the separator and the shield. 

In regard to the optical bar mount, the present bar 
is supported at two places by three radial members, 
in the Ml Height Finder. A taper pin through one 
member prevents rotation and slipping of the bar. 


restricted\ 


QUARTZ-COPPER PENTA-REFLECTOR ASSEMBLIES 


53 


This type of mount is supposed to support the op- 
tical bar free from all restraints, but the present tests 
showed that this was not the case. Mechanical 
stresses were transmitted to the bar by friction be- 
tween it and the supports, and through the taper pin. 
The use of a kinematic mount, described in the text, 
was investigated, but it was not practical because of 
the flexibility of the optical bar. A mount giving the 
bar freedom of motion relative to its supports was 
developed in which the bar was supported by cast- 
iron rings, with a minute clearance between the rings 
and the support. The bar was kept in position by a 
pin, the end of which was turned to a sphere. This 
mount is very similar to the original Keuffel and 
Esser design. An Ml Height Finder was fitted with 
this type of mount and subjected to an ambient tem- 
perature change of 70 F. Range errors produced by 
the optical bar mount under these conditions could 
not be detected. 

An Ml Height Finder was equipped with these 
modifications, consisting of the penta-reflector blocks 
covered with flexible copper braid and the optical 
bar mounted in the cast-iron rings described above. 
The modified height finder was subjected to an ambi- 
ent temperature change of 70 F and the range errors 
determined by the internal adjuster. It was found 
that range errors resulting from the penta-reflectors 
were reduced from about 10 to 4 UOE by the copper 
shielding. Those originating in the optical bar 
mount were eliminated by the change in mount. A 
slow drift of about 10 UOE over a period of about 
12 hours remains. This was originally attributed to 
the optical bar mount, but depends rather on the 
temperature of the height finder in a manner not yet 
determined. Thus, the range errors in the present 
Ml Height Finder can be materially reduced with 
rather simple modifications. 


The report also indicates the nature and magni- 
tude of these errors. When Ml Height Finders are 
subjected to ambient temperature changes of 60 to 
70 F, the RCS values, as determined by the internal 
adjuster, follow a fairly definite pattern. The RCS 
starts to drift about 15 minutes after the temperature 
change, reaches a maximum shift of about 10 UOE 
in 1 to 2 hours, and then returns to approximately 
its original value after 4 to 5 hours. This short-time 
RCS shift is found in about the same amount in all 
height finders. Superimposed on this shift is a second 
long-time drift which starts about two hours after the 
temperature change and continues for 8 to 10 hours 
often reaching a maximum of 30 to 40 UOE. The 
nature of this long-time drift depends on the par- 
ticular instrument and may be either positive or 
negative. The experiments described above using 
end blocks of different materials and with optical 
bars mounted in various ways showed that the short- 
time shift of 10 UOE is produced by a change in the 
angle of the penta-reflector, while the long-time 
error results from strains in the optical bar produced 
by stresses transmitted through its supports. This 
might be expected since the penta-reflector mount is 
the same in all height finders and any errors arising 
from it should be the same in all instruments. On the 
other hand, mechanical stresses transmitted to the 
optical bar through its mounts will depend on the 
position of the optical bar in its mounts, and hence 
may vary from one height finder to another. 

These several studies on temperature effects on 
penta-reflectors and optical bars are gathered to- 
gether in a Report to the Services issued by the Fire 
Control Division of NDRC. (41) Inasmuch as no 
further research in this field is contemplated, this 
may be considered as a final report on these matters 
from this source. 




RESTRICTED 


Chapter 6 

POWER AND BASE LENGTH 


6 1 MAGNIFICATION 

A comp.\rison of 12 and 24 power in ranging on 
fixed targets is reported by the Fort Monroe 
Princeton Laboratory. (344) The purpose of the 
study was to deteiTnine the comparative reliability of 
range determinations at the two powers provided in 
the Ml and M2 Height Findei's; the comparative re- 
liability of internal adjuster readings at these differ- 
ent powers and the net correction to RCS (Curve B) 
at different powers. Six experienced observers and 
three Ml and one MJ instruments were used. The 
results show that the ratio of 12-power discrepancy 
between duplicate readings to that of 24 power, over 
the test period of almost a month, was about 1.10. 
No significant corrections between the ratio and the 
individual man, the particular target, or the day 
could be established. The reliability of internal ad- 
juster readings appeared to be about the same for 
both powers. This is somewhat surprising; it appears 
to indicate that factors other than pure visual per- 
ception of angles play an important part in the in- 
ternal adjuster setting. Also, for most observers, the 
net correction to RCS is about the same for 12 as 
for 24 power. However, two of the better observ ers 
showed considerable difference between 12 and 24 
power: the size of this difference was opposite for 
the two men. No explanation of this anomalous be- 
havior is advanced. 

Another set of experiments was performed by the 
Princeton Laboratory at Fort Monroe using aerial 
targets. (359) Height readings were taken on aerial 
targets with six standard Ml Height Finders using 
four combinations of power and aperture. Magni- 
fications of 12 and 24 power were combined in all 
possible ways with 1-inch aperture and the normal 
2.5 aperture of the wide-open instrument. Consist- 
ency at reduced aperture was found to be much 
better than at full aperture, using either 12 or 24 
power. Precision with 24 power was not twice as 
good as that with 12 power, which ratio is theo- 
retically expected when the obser\’er’s sensitivity is 
the controlling factor. These findings agree with 
those already noted for ranges taken on fixed ground 
targets using reduced power or reduced aperture 
separately. For example, with 1-inch aperture the 
average precision errors were 2.7 UOE for 24 power 


and 3.2 UOE for 12 power, and for the full- field in- 
strument, 2.7 UOE for 24 power and 4.1 UOE for 
12 power. The consistency errors were, for 1-inch 
aperture, 5.4 UOE with 24 power and 8.5 UOE with 
12 power and, for the full field, 5.4 UOE at 24 power 
and 3.9 UOE at 12 power. Thus, the observer con- 
sistency is markedly worse than when both power and 
aperture are reduced. This is the combination giving 
an especially short eye distance, the distance of the 
exit pupil from the ocular. 

The results of these two experiments are presented 
in a unified report of the Princeton Laboratory at 
Fort Monroe with the addition of other material on 
fixed targets. (366) The analysis shows that the pre- 
cision, measured in yards of error, of readings taken 
with the Ml Height Finder was substantially better 
at 12 than at 24 power for range observations on fixed 
ground targets and was substantially better at 24 
than at 12 power for height observations on aerial 
targets. The relative precision of observations taken 
at the two magnifications varied greatly from ob- 
server to observer, but not from instrument to instru- 
ment nor for different target distances. The ratio 
of precision error in yards at 12 power to that at 24 
power, for the 35 observers in this test, ranged be- 
tween 0.40 and 1.36 for fixed targets and between 
0.64 and 3.67 for aerial targets. These ratios for the 
middle 19 of the 35 observers ranged between 0.63 
and 0.90 for fixed targets and between 1.17 and 1.73 
for aerial targets. The average ratio was 0.79 for 
fixed targets and 1.47 for aerial targets. 

This document is attached as supporting data to 
a Report to the Serv ices issued by the Fire Control 
Division of the NDRC. (19) The experimental data 
indicate that, under none of the circumstances tested 
was the precision error at 12 power, when measured 
in yards, twice as great as at 24 power, as would be 
predicted from the simple theory of geometrical 
optics. On aerial targets the ratio was more nearly 
1.5; perhaps slightly greater at short ranges and 
slightly less at long ranges. That is, only about half 
the theoretical advantage of 24 power was realized. 
On fixed targets the ratio was actually less than 1; 
that is 12 power gave more precise readings in yards 
than 24 power. This was unexpected and the reason 
for it is not definitely established. It is probably 
associated with the atmospheric conditions at the 


(restrictkd 


54 


EFFECT OF MAGNIFICATION ON PRECISION 


55 


time of the tests, though the subjective judgments of 
the conditions as noted down by the experimenters 
do not support this inference. However, if there were 
heat waves or atmospheric boiling between the in- 
strument and the ground target, this would be mag- 
nified greatly when 24 power was used and well 
might affect the results. The possibility is suggested 
that an intermediate fixed power (say 18) might be 
preferable to the present variable power. The evi- 
dence is not conclusive enough, however, to justify 
a recommendation to that effect. 

To test this suggestion further an eyepiece assem- 
bly with 18 and 36 power was provided and tests 
were made at Camp Davis by the Board com- 
paring the full range of 12, 18, 24, and 36 power. 
These data have been taken but up to the present 
(September, 1944) have not been calculated, ana- 
lyzed, or reported. 

The Princeton Laboratory has reported a number 
of individual studies on power made at Fort Monroe 
which are largely the basis of their several more 
formal reports. The first is a preliminar)' report on 
the effect of change of power on the spread of range 
finder readings on fixed targets. (462) The repro- 
ducibility of range finder determinations on fixed 
targets with the two powers is reported in another 
study. (463) Another paper deals with the relation 
of RCS and power. (464) Were the visual angle alone 
in control, it would be expected that the standard 
deviation of settings at 24 power would be half that 
at 12 power. Such is not the case. Indeed the question 
may be raised as to whether there is any evidence 
that the ratio differs from unity. Of some 15 ratios 
for 8 observers only one is statistically significantly 
different at the 5 per cent level. There is a preponder- 
ance of ratios greater than 1—12 out of 15. The range 
of the ratios runs from 0.68 to 1.32. Another study, 
dealing with net correction to RCS as effected by 
power, indicates that five of the seven observ ers show 
substantially the same net correction for both 12 and 
24 power. (465) 

A preliminary study is reported on the comparison 
of 1 2 and 24 power on aerial courses, which gives in 
more detail the aerial data summarized above. (466) 
A final Fort Monroe Princeton Laboratory report 
deals with the effect of power on the stability of 
range readings on fixed targets. (467) Here it was 
discovered that the average of the mean absolute 
deviations was less for 24 power when either clear 
visibility or haze was reported, and less for 12 power 


when heat waves were reported. It is also pointed 
out that there may be an instrument factor which 
makes for either longer or shorter ranges on 12 power 
as compared with 24 power. All observers read 
shorter on 12 power on a particular height finder, 
nearly all shorter on 12 power on another, and nearly 
all longer on 12 power on a third instrument. It is 
possible, therefore, that some factor peculiar to the 
individual instruments may be responsible for a 
changed level of range readings when the power is 
changed. 

A statistical study of the precision of a stereoscopic 
range finder upon the magnification employed is 
reported by the Applied Mathematics Panel of 
XDRC. (66) The data utilized in this study consist 
of acceptance-test records available in the files of the 
Naval Inspector of Ordnance-Optical Materials. The 
records utilized related to instruments manufactured 
by the Bausch and Lomb Optical Company and those 
by Keuffel and Esser Company. In each instance, the 
records were those of the tests carried out by the staff 
of the Naval Inspector of Ordnance and are not the 
inspection records of the company’s inspectors. The 
present analysis deals with the records of the Mark 45 
Stereoscopic Range Finder and involves the inspec- 
tion data on 39 instruments. The results indicate 
that the precision of the instrument when 24 power 
w^as employed to the precision when 12 power was 
used was in the ratio 1.22 to 1. According to the 
theory of geometrical optics, these precisions should 
be in the ratio of 2 to 1. Thus it would seem that 
under the observing conditions at the Bausch and 
Lomb plant, an increase of magnification from 12 to 
24 power resulted in an increase of precision which 
w'as roughly one-fifth of the increase expected from 
the theory of geometrical optics. 

6 2 base LENGTH 

Another report of the Applied Mathematics Panel 
of NDRC is concerned with the dependence of the 
precision of stereoscopic range finders on base length. 
(67) This again is based on a statistical analysis of 
acceptance-test records for stereoscopic range finders 
varying in length from 18 to 46 feet. The analysis 
suggests that, under acceptance-test conditions, the 
precision increases with base length. The rate of 
increase seems to depend on the observer to a certain 
extent. However the results again indicate a break- 
down of the theory of geometrical optics, according 


[RESTRICTED 



56 


POWER AND BASE LENGTH 


to which the precision of a stereoscopic range finder 
should be proportional to the base length. From an 
examination of the inspection records relating to the 
Mark 45, Mark 37, Mark 46, and Mark 52 Stereo- 
scopic Range Finders, which have base lengths of 18, 
26.5, 43, and 46 feet respectively, it appears that the 
precision is not up to theoretical expectation. 

A laboratory study at Harvard University was 
made under laboratory conditions in which it was 
found that the full theoretical effect of neither mag- 
nification nor base length was realized in actual ob- 
servation. (267) The readings in this study were all 
on fixed targets under artificial, but excellent, labora- 
tory conditions. 

63 EFFECT OF MAGNIFICATION ON 
STEREOSCOPIC ACUITY 

Further experiments at Harvard University have 
to do in part with the problem of magnification and 


its effect on stereoscopic acuity. (283) These are sum- 
marized in some detail in Chapter 2, Fundamental 
Studies. Both laboratory and field studies were con- 
ducted— the latter over both land and water. Magni- 
fications were employed from lx (the unaided eye) 
to 40x. Ranges varied from 50 yards in the laboratory 
to 6,400 yards over water. The results of all of these 
studies are similar. The relation found between mag- 
nification and stereo acuity indicated that the angu- 
lar error at the eye was not constant, but increased 
in direct proportion to the increase in magnifying 
power. Expressed in per cent units (=100xAR/R) 
the error was constant and independent of magnify- 
ing power. Moreover, the per cent error of the ob- 
servations was unexpectedly low. For any of the 
ranges or magnifying powers employed, the average 
mean — variation and the mean — variation of the 
average adjustments was always less than 1 per cent. 
Experiments leading to the explanation of these find- 
ings are summarized in Chapter 2. 


RESTRICTED 


Chapter 7 

CALIBRATION OF RANGE FINDERS 


71 INTRODUCTION 

M ost of the range and height finders in service at 
present are provided with adjusting systems de- 
signed to correct for any changes in optical alignment 
which may occur after the initial assembly and ad- 
justment. These changes may be caused by movement 
of lenses in their mounts, by bending of the optical 
bar under stress or even temperature conditions, or 
by other causes. The method of adjustment depends 
essentially on the use of an artificial target provided 
within the instrument and designed to appear at in- 
finity when the instrument is in proper optical align- 
ment. An adjusting wedge is provided so that the 
alignment may be corrected as necessary. These ad- 
justments must be made quite frequently under 
usual conditions and any errors made in these set- 
tings are reflected directly in subsequent range and 
height readings. 

7 2 VARIABILITY OF RANGE 

CORRECTOR SETTINGS 

During the work of the Princeton Laboratory at 
Fort Monroe a considerable mass of data was ac- 
quired which indicated how the Range Corrector 
Setting [RCS] varied during a series of observations 
and how inaccurate these settings were when made 
by student or even expert observers. A number of 
studies (376, 377, 378, 379, 380, 381) indicate that 
these RCS readings are not consistent either as to 
the means or the variances. In most cases, 10 RCS 
readings were made by each observer on each instru- 
ment before range finding readings were taken and 
a similar number of RCS readings were made by each 
observer following his ranging. These two sets of 
10 readings for each man-instrument combination 
were examined for differences of means and vari- 
ances. The set of variances for each man on all instru- 
ments was tested for homogeneity. In one case (376) 
it was found that 12 out of 25 pairs of means differed 
significantly. Five of the 12 pairs were from a par- 
ticular instrument. Four out of 25 pairs of variances 
were found to differ significantly. Neither the entire 
set of variances of RCS readings before ranging, nor 
the entire set of variances of RCS readings after 
ranging, was homogeneous. Only two of ten observers 


obtained sets of variances on all instruments before 
ranging which were homogeneous and two men ob- 
tained homogeneous results after ranging. Only one 
observer gave homogeneous results both before and 
after ranging. 

In another experiment (380) two groups of five 
teams observed alternately for a total of eight courses, 
each group observing on four courses and each team 
observing on the same instrument. Each team made 
10 readings for P>.CS start and finish using the same 
type of contact throughout. The results indicate 25 
out of a total of 40 pairs of means showed that the 
RCS finish was significantly different from the setting 
of RCS at the start of the courses. Of these 25, 15 had 
the larger mean on RCS finish. Two observers 
showed significantly different means on all four 
courses while only one observer showed no signifi- 
cant difference in means for any course. 

These results were obtained in spite of the fact 
that only one of the ten observers obtained variances 
for RCS start and finish which were not homogene- 
ous and for this observer this was only true on RCS 
finish. 

The Princeton Laboratory at Fort Monroe has 
analysed the results of a number of experiments to 
determine the limits of accuracy of RCS settings and 
their influence on ranging. In one study it was noted 
that even the best observer had a standard deviation 
of 0.75, and the worst a standard deviation of 1.78. 
From these results the investigators believe that 
one must expect a standard error in RCS setting 
of about 1 to 1.25 units. (382) If one assumes that a 
similar variability will affect a range finder reading, 
one can say that the minimum standard deviation to 
be expected in such ranging determinations would 
be of the order of 1.4 to 1.7 UOE, using the pro- 
cedure of an RCS determination by the observer 
before each ranging determination. Another study 
had to do with variation of RCS due to observer, 
instrument, and method of contact but the results 
are not clear-cut and are difficult to interpret. (383) 
Another report considers RCS settings, namely, net 
correction to RCS under constant temperature. (384) 
Two expert observers and two instruments were 
used. The data obtained were similar to those char- 
acteristic of former experiments. It appeared that 


RESTRIcTdi^ 


57 


58 


CALIBRATION OF RANGE FINDERS 


both instruments gave a positive error— of 20 UOE 
and 10 UOE respectively— although both were given 
a wedge check just before the start of the experiment. 

’•2 * Factors Affecting RCS Accuracy 

Another Princeton Laboratory report deals with 
the influence of errors in RCS on ranging errors. 
(385) This study was made before the full complex- 
ities of the temperature problem were realized. The 
mean change in RCS per degree F change in ambient 
temperature for four instruments was found to vary 
between —0.40 and —0.83, with standard errors 
varying between 0.15 and 0.27 scale divisions. One 
M2 instrument differed considerably from the three 
Ml instruments, a difference which is believed to 
be due to differences in construction. A further 
analysis of instrument and observer differences will 
be found in another report. (386) 

The effect of internal target position on RCS is 
reported. (387) Five target positions of the internal 
adjuster target of an M2 instrument covered the 
distance on the reticle pattern from the two small 
pegs to the left of the center post to the two small 
pegs to the right of the center. When the central 
setting was used, the two lines of the target fell one 
on each side of the center reticle mark. Three sub- 
jects made observations— each making 200 readings 
during a single morning. All settings were made with 
the target approaching the observer on the last move- 
ment of the permitted bracketing. It was found that 
the average scatter for the central positions was less 
than for each of the other target locations for all 
observers. For individual observers, however, other 
positions were often as effective. The average differ- 
ences are not great, however, being 55.06 for the 
central position and 54.42 and 56.31 for the two 
extreme positions of the target. It is pointed out that 
the use by an observer of any target position other 
than the central one in determining the RCS intro- 
duces an additional factor in Curve B, but this factor 
will be constant from day to day only if the adopted 
target position is likewise constant. 

A comparison of binocular and monocular RCS 
settings is reported by the Fort Monroe Princeton 
Laboratory. (388) An M2 instrument equipped with 
collimator scale was used. Four observers were used. 
Significant differences were found for the binoculai 
and the monocular procedures, right and left eye. 
It was found that the observer who brings the target 


nearest for RCS binocularly moves the collimator 
knob less establishing the monocular enclosure when 
either the right or left eye is stimulated. The final 
short study reported by this group has to do with 
the accuracy of single RCS determinations. (389) 
Pairs of observers on four instruments gave standard 
errors which ranged from 0.90 to 1.67 UOE. It should 
be noted that the values of both observers with an 
M2 instrument were larger than those of any of the 
other six observers with three Ml Range Finders. 

As a result of these experiments, the Princeton 
Branch of Fort Monroe suggest the use of external 
targets as the basis of making range finder adjust- 
ments. (348) It makes specific suggestions for the 
modification of present range finders which, it is 
believed, should lead to a considerable improvement 
in their overall performance by the substitution of 
an external adjuster system. It is unfortunate that 
in the instruments now in service the internal target 
is so different from the one the observer is concerned 
with under combat observation. Of course, this is 
necessarily true of any artificial target. The system 
suggested would permit the use of external targets 
of unknown range in making the infinity adjust- 
ment. To accomplish this it would be necessary to 
remove the internal adjuster system except for the 
adjuster wedge and the penta-prism assemblies and 
to replace the right penta-prism by a prism which 
would split the beam, permitting one half to pass 
through the right optical system in the usual man- 
ner, reffecting the other half at a right angle along 
the present internal adjuster path so as to pass finally 
through the left optical system. The infinity adjust- 
ment would be made, using any external target de- 
sired or available, by setting the adjustment wedge 
so that the range reading for this target is infinity. 
This procedure is simply the “known range” method 
where the target provided is at infinity. This external 
adjuster method does not immediately eliminate all 
of the faults of the usual internal adjuster system. 
For example, refractive errors due to temperature 
gradients in the gas with which the instrument is 
charged will persist. The following advantages are 
claimed for the proposed system: (1) The adjust- 
ment is made using the same criterion for the stere- 
oscopic setting of adjustment as is used while actually 
ranging, thus eliminating all personal and instru- 
ment errors caused by the peculiarities of the par- 
ticular observing conditions encountered at any 
time. (2) Any external target may be used for prac- 
tice and training with the range finder without need 



PRECISION OF SETTINGS 


59 


for triangulation of target course to determine the 
quality of the observer’s performance. (3) The pro- 
duction and assembly of instruments would be sim- 
plihed by the elimination of the present rather com- 
plicated internal adjuster system. (4) The proposed 
system may be used equally well with coincidence, 
ortho-pseudo and direct stereoscopic and other types 
of range finders. (5) Relatively simple training in- 
struments may be constructed using the proposed 
external adjuster method, which simulates the range 
finder task very well. 

A hnal study of calibration of range hnders by the 
Fort Monroe Princeton Laboratory considers the 
elimination of the need for frequent adjustment of 
these instruments. (349) This report discusses the 
instrumental errors inherent in the M 1 Stereoscopic 
Height Finder and proposes the application of range- 
finding design which would eliminate most of them. 
The essential notion is that satisfactory precision can 
be built into a range finder and in such a way that 
frequent calibration and readjustment will not be 
necessary. This report presents a principle of optical 
arrangement which offers an important gain in 
accuracy by excluding those stray effects now merely 
compensated for by frequent reference to a standard 
range and introduction of a range corrector setting. 
The application of this principle involves no new 
range-finding method and is equally suited to a stereo 
reticle, coincidence, or ortho-pseudo comparison 
fields. The Ml Stereoscopic Height Finder, for ex- 
ample, employs an accessory internal adjuster to 
provide a primary range standard for determining 
corrections to a working range standard built into 
the main optical system. Frequent adjustment of the 
correction value is required because of stray effects 
due partly to bending of the structure and light rays 
which form the working standard. This study urges 
the desirability of arranging the main optical path 
so that it embodies a working standard as simple 
and free from stray effects as the best primary stand- 
ard, thereby obviating the internal adjuster and 
range corrector setting. Various sources of error and 
means of eliminating them are discussed. 

7.2.2 Precision of Internal Adjustment 
Settings 

The Army Height Finder 

The Applied Psychology Panel has released two 
reports dealing with calibration procedures and 
errors for the Army and Navy instruments. The first 


of these is concerned with the calibration of Army 
height finders. (79) This report presents an analysis 
of the calibration records of several groups of stu- 
dents in classes at the Height Finder School at Camp 
Davis. It is pointed out that hnding the right calibra- 
tion correction to use is one of the most important 
jobs a new operator must learn. The height finder 
is so constructed and the theory of its operation is 
such that an observer who is highly skilled and keeps 
his instrument in perfect adjustment is able to meas- 
ure true heights of aerial targets, true ranges to aerial 
targets, or true ranges to ground targets using the 
same calibration correction for all three conditions 
of observation. The records of students at the Height 
Finder School show that a good student with 11 
weeks of training on stereoscopic trainers and height 
finders makes good height or range readings on aerial 
targets using the same calibration correction. His 
error of height readings increased when he used a 
fixed target calibration correction for aerial targets 
instead of a specially determined aerial target cali- 
bration. They increase by a factor of at least 3 if his 
instrument is in poor adjustment and by a factor of 
about 2 when his instrument is in good adjustment. 
This error can be reduced still further by practice in 
stereoscopic observation on both ground and aerial 
targets. In one winter class it was found that the 
optimal corrections for fixed targets and for aerial 
courses differed by 4 UOE for 70 per cent of the men. 
The average difference for the 37 men was zero. 
These findings apply equally to the use of the 
“known range” method or the use of a celestial 
target in making his calibration correction. These 
results lead to the following recommendation. The 
Army Height Finder School uses a special calibrating 
procedure which allows the student to determine on 
aerial target calibration correction by comparing his 
readings with target position data obtained by a 
target Practice Record Section using phototheodo- 
lites. To emphasize the importance of proper in- 
strument maintenance and to give men satisfactory 
training in calibrating the height finder under field 
conditions, it is recommended that students at the 
Height Finder School be trained using a calibration 
correction based on fixed target and on celestial 
target readings instead of a calibration correction 
based on Record Section measurements. 

The Navy Mark 42 Range Finder 

The second report deals with precision of internal 
adjustment settings of the Navy Mark 42 Range 


60 


CALIBRATION OF RANGE FINDERS 


Finder. (80) Although experienced range finder 
personnel consider that a series of internal adjuster 
settings should have a spread of less than 1 UOE, 
no experimental studies had been undertaken to 
determine the precision with which Naval personnel 
actually make these settings. The present research 
was undertaken to determine the range of internal 
adjuster settings made by student range finder oper- 
ators using the usual stereoscopic method at the 
Fire Control School at Fort Lauderdale. It was found 
that 18 advanced range finder students made a series 
of five internal adjuster settings in the course of 
routine operations with an average range of 2.50 
UOE and an average standard deviation of 1.10 
UOE. To ascertain whether students make more 
precise readings in an experimental situation where 
greater care was used in recording the settings, six 
men made a series of stereoscopic internal adjuster 
settings. Although the mean range and mean stand- 
ard deviation of this experimental group of 6 men 
were less than the same measures for the 18 men, 
a significant difference was not demonstrated. In 
the case of the 18 men the range of means varied 
from 0.5 to 10.0 UOE and only 5 men had 1 UOE 
or less while 7 men had 2 UOE or more. An addi- 
tional portion of the experiment was devoted to a 
comparison between stereoscopic and monocular 
methods of making the internal adjustment. A sig- 
nificant difference in precision between the two 
methods was not demonstrated. Recommendations 
for training procedures designed to produce more 
precise internal adjustment by range finder operators 
are given in detail, indicating the steps to be taken 
in making a series of 9 settings— the median value to 
be used as the internal adjustment correction. 

A further report by the Applied Psychology Panel, 
NDRC, is an extension of the previous study. (97) 
The precision of internal adjustments made by stu- 
dent operators following the revised procedure was 
measured at various times during the period of train- 
ing at the Naval Training Station at Fort Lauder- 
dale. In order to establish a standard for comparison 
and to determine the degrees of precision attained 
by experienced shipboard range finder operators, 
records of series of internal adjustments made by 
such operators were obtained and analyzed. It was 
found that 81 student operators of Mark 42 Range 
Finders attained a mean range or spread of internal 
adjustment settings of 2.27 UOE. This smallest range 
was reached during the sixth week out of eight weeks 


of training on this instrument. Learning, as meas- 
ured by an increase in precision in making internal 
adjustments, occurred from the first to the sixth 
week. Twelve experienced shipboard range finder 
operators, each making 63 to 75 series of internal 
adjustment settings, obtained a mean range of 1.37 
UOE. This figure is of significantly greater precision 
than that obtained by the student operators. The 
revised procedure in making internal adjustments 
enabled the student operators to increase the preci- 
sion with which they made the settings. In view of 
these experimental findings, the investigators recom- 
mend that additional stress be placed upon internal 
adjustment setting procedures and that measures be 
taken to increase motivation with regard to this 
adjustment. The settings should always be measured 
at least to the nearest 0.25 UOE. Motivation might 
be increased by periodically posting, in a conspicu- 
ous place, the range or scatter of series of internal 
adjustments made by all the student operators in a 
given class. 

The Applied Psychology Panel reports an experi- 
ment carried on at the Fire Control School at Fort 
Lauderdale, Florida, comparing five methods of 
calibrating the Mark 42 Range Finder. (89) A range 
finder operator must compensate for his personal 
error in ranging by applying a calibration correction 
to his instrument. Only by so doing can he be ex- 
pected to obtain accurate ranges. There are a num- 
ber of methods by which an operator can obtain his 
calibration correction. This experiment was de- 
signed to compare the magnitude and precision of 
calibration corrections obtained by as many methods 
as were feasible at a shore installation although every 
attempt was made to duplicate, insofar as possible, 
the way in which these methods would be used 
aboard ship. 

The 12 best operators in a training class obtained 
calibration corrections by ranging with the correc- 
tion knob on celestial targets: a star, the moon, and 
on a fixed target of known surveyed range; with the 
correction knob method and with the range knob; 
and by ranging on an aerial target simultaneously 
with fire control radar. Also calibrations were made 
by ranging with the correction knob on the sun when 
its brightness was reduced by rhodium mirrors fitted 
over the end windows. 

The following results were derived from a statisti- 
cal analysis of the data. The magnitudes of the cali- 
bration corrections obtained by each of the methods 


RESTRICTED 




COMPARISON OF INTERNAL TARGETS 


61 


do not differ significantly from each other. The 
methods rank as follows in terms of precision: (1) 
known range method (correction knob); (2) known 
range method (range knob); (3) celestial star target, 
celestial sun target and radar method. When oper- 
ators are given sufficient practice in the use of the 
celestial target methods, their precision should ap- 
proach that of the known range methods, ft is 
pointed out that, in terms of adaptability to ship- 
board conditions, the celestial target methods and 
radar method are superior to the known range meth- 
ods. The report recommends training in all these 
methods with an emphasis on greater proficiency 
than has been required hitherto, ft is also recom- 
mended that rhodium mirrors or suitable filters be 
provided for all range finders to make possible the 
use of the sun as a celestial target. 

7.2.3 The Internal Adjuster Target 

Single and Parallel Bars 

A number of studies have been made of the arti- 
ficial target employed in the internal adjuster system 
of range finders. One study by Ohio State University 
was a direct experimental attack upon the question 
of the accuracy and precision of internal adjuster 
settings, using a single bar or the present double bars 
as the reticle to which the diamond target is to be 
stereoscopically adjusted in the Naval instruments. 
(324) It was found that so far as precision of settings 
is concerned, there is essentially no difference be- 
tween using two bars and using one— average values 
being 0.442 and 0.425 UOE respectively. There was 
some indication in the results that tilting of the 
horopter causes a constant error in the average set- 
tings of the diamond when a single bar to the right 
or left was used as reference point, but is not mani- 
fest in the case of the iwo-bar target. The error aris- 
ing from this source was found in one case to be 
approximately 0.447 UOE. Data from a limited 
number of cases indicate that better results are ob- 
tained using black figures on a light background 
than white figures on a dark background. The results 
of several subjects who failed to make precise set- 
tings showed a marked tendency to set the diamond 
at a considerable distance in front of the bars. The 
explanation of this effect was not discovered, but it 
points to the conclusion that large constant errors 
can be expected if the subjects are not able to make 


precise settings, and that these errors will, in general, 
be in the direction of setting the diamond in front 
of the target bars in order to make them appear in 
the same place. 

Vertical Line and Other Patterns 

A series of experiments dealing with reticle and 
artificial target patterns for the internal adjuster 
was made by Brown University. In the first of these 
reports five patterns for use in internal adjusters for 
monocular instruments were tried and one was found 
to be superior to the other four for both experienced 
and inexperienced observers. (139) This pattern, 
which consists of a longer vertical line to be placed 
so that it bisects the space between two smaller verti- 
cal lines, yields more precise and more consistent 
range corrector settings. With this pattern and under 
favorable conditions, an average experienced ob- 
server is able to maintain a level of precision which 
is represented by an average deviation of 3.2 seconds 
of arc, or 0.27 UOE from the mean of his own set- 
tings. Under similar conditions, this pattern yields 
a consistency of mean settings from observer to ob- 
server as represented by an average deviation of 3.2 
seconds of arc or 0.27 UOE from the mean of all 
settings made by a group of observers under com- 
parable conditions. Inexperienced observers are 
nearly as consistent as experienced ones and the prac- 
tice effect from the first to the second day of experi- 
mentation is negligible or nonexistent. With this 
pattern the mean average deviation of 25 inexpe- 
rienced observers was 0.31 UOE as compared with 
0.27 UOE for the experienced observers as noted 
above. This pattern has an accuracy and a consist- 
ency at least 20 per cent or more better than any of 
the four other patterns which were centering a dot 
inside a circle, bisecting with a vertical line the space 
between two diamonds, bisecting an open diamond 
with a vertical line, and bisecting a circle with a 
vertical line. Finally the superiority of this best pat- 
tern was maintained over most of the range of useful 
brightnesses for the light-adapted eye from 0.3 to 30 
millilamberts. 

The second Brown University experiment is a 
more complete study of two patterns— the fiducial 
line between the target of two vertical lines and be- 
tween two target diamonds— under six levels of illu- 
mination varying from 129.0 to 0.033 apparent foot- 
candles. (142) It was found, for both types of target, 
that precisions are poor at low illuminations and 


RESTRICTED 


62 


CALIBRATION OF RANGE FINDERS 


become better as the light intensity increases. The 
target line proved to be superior to the diamond 
target at all but the very lowest level of illumination. 

In an additional Brown University experiment, 
using only the vertical line target, an attempt was 
made to determine the effect of the separation of the 
two vertical target lines which determine the space 
to be bisected by the fiducial line. (143) Six separa- 
tions were employed equivalent to angular separa- 
tions of 5.5 to 22.1 minutes of arc respectively, all the 
lines remaining at 5.5 minutes of angular width. The 
results show that precision is greater for the smaller 
separations and precision decreases approximately 
linearly as the separation between the target lines 
increases. 

These experiments were made with a testing 
apparatus devised at Brown University to study 
internal adjuster settings of either the monocular 
(vernier) or binocular (stereoscopic) types. This 
apparatus is described in detail. (158) When com- 
parable visual patterns were used for stereoscopic 
and monocular performance, monocular settings 
were made with somewhat greater precision and with 
smaller constant errors than stereoscopic settings. 
In another series of experiments at Brown Univer- 
sity, it was found that the highest precision was 
obtained when a vernier bisection method was used 
with monocular vision on an internal adjuster pat- 
tern involving the centering of a vertical line be- 
tween two fiducial lines. (159) As a result of these 
experimental findings, the investigators suggested 
that the precision of internal adjuster settings may 
be increased over the present method by employing 
monocular observation, as in the so-called Abbe in- 
ternal adjuster system, instead of attempting to make 
stereoscopic settings. Greater constancy of settings 
between observers was also found for the monocular 
system. It is pointed out that, besides the fact that 
the Abbe system gives greater precision of setting 
the internal adjustment of the instruments, its adop- 
tion would have other advantages. These include: (1) 
utilization of the normal path for internal adjuster 
settings; (2) considerable reduction in the number 
of optical parts required; (3) simplicity of construc- 


tion; (4) relatively greater clearness of the internal 
adjuster target images; and (5) less susceptibility to 
the effects of temperature and other external influ- 
ences. Two possible schemes for applying these find- 
ings in the present types of range finders are indi- 
cated. One of these involves placing internal adjuster 
targets on the existing reticles used for ranging; the 
other involves a vernier setting on the present un- 
changed reticles of the Army Ml and M2 Height 
Finder instruments. 

These five documents are attached to a Report to 
the Services issued by the Fire Control Division of 
NDRC. (31) This report makes the following recom- 
mendations: (1) The adoption for monocular range 
finders, particularly of the short base type, of an in- 
ternal adjuster pattern which involves setting a fidu- 
cial line midway between two target lines; (2) The 
introduction of a monocular type of internal adjuster 
system into one of the present stereoscopic range 
finders, both laboratory and field tests to be given 
this modified instrument. The Naval Bureau of Ord- 
nance had this modification made in a 1.5-meter 
stereoscopic range finder by having the Naval Gun 
Factory remove the normal internal adjuster system 
and introduce the Abbe system of calibration. This 
modified instrument and a normal instrument have 
been sent to Brown University for comparative study. 

Brown University has completed one more study 
which embodies the results of a further analysis of 
the precision of range corrector settings for small 
monocular range finders. (136) There were 6 expe- 
rienced and 25 totally inexperienced operators and 
three brightness levels of illumination were used. 
Only two patterns were used, namely, placing the 
vertical fiducial line midway between two target dia- 
monds or two smaller vertical target lines. The re- 
sults are the same as before, showing a higher degree 
of precision and better agreement among observers 
for the line target pattern. This is true for both 
skilled and unskilled observers and for three degrees 
of brightness (from 0.3 to 30 millilamberts). Even 
with unskilled observers the distribution of mean 
settings on this pattern shows an average deviation 
of less than 0.3 UOE from one to another. 


c 


RESTRICTED 


a 


Chapter 8 

MISCELLANEOUS INSTRUMENT AND 
OPERATIONAL DEFECTS 


8 1 INSTRUMENTAL CAUSES OF RANGE 
FINDER ERROR 

8.1.1 Penta-System Rotation 

A FORT MONROE PRINCETON LABORATORY report clis- 
cusses the height finder errors which might be 
caused by penta-system rotation. (5 1 3) This may occur 
in any one of three planes. The report presents the 
hypothesis that a certain type of distortion may be 
expected in a height finder which would introduce 
range errors in the top and bottom of the field. There 
is a small amount of experimental evidence that such 
errors actually occur. The paper also contains the 
calculation of the associated second-order effects. 
The only first-order effect occurs with rotation about 
one particular axis and would produce an apparent 
slant back or forward in the stereo position of a verti- 
cal object. This effect would amount to 0.0204 UOE 
disparity at top or bottom of the field for 1 UOE 
rotation of the penta-system with 24-power magni- 
fication. At 10,000 yards range a rotation of the image 
in the field of 0.018 degrees would produce a range 
error of ±100 yards. It is believed that such play in 
the penta-systems is possible in field service. 

Type of Filter 

An experiment run at Fort Monroe and reported 
by the Prinecton Laboratory indicates that real dif- 
ferences in performance with different filters, under 
ordinary atmospheric conditions, are probably not 
more than 2 UOE in size, if they exist at all. (468) 
Five filters are available in the Ml Height Finder; 
clear, amber, red, dark, and blue. There is a slight 
indication that the use of a clear filter (the filter 
most frequently used) may result in a somewhat 
smaller consistency error than results from the use 
of the other filters. This effect is about 1 UOE in 
magnitude, a reduction by a factor of nearly 2. There 
was no evidence of any other filter effects. These 
conclusions are based upon 60 range observations 
with each of the five filters by two experienced test 
observers. 


8.1.3 Leveling and Alignment 

A theoretical study of the severity of the systematic 
height and range errors in an Ml Height Finder as 
a result of known leveling and alignment errors is 
reported by the Princeton Branch of the Frankford 
Arsenal. (243) The memorandum discusses the prob- 
lem of alignment and index errors in this instrument, 
with particular reference to the required accuracy 
of leveling and wedge-check agreement and to the 
desirable changes in specifications and field proce- 
dures. The conclusions apply immediately to any 
13.5-foot base, 24-power height finder used with a 
data transmission system of accuracy comparable to 
that used in the Ml Height Finder, provided height 
is not read below 200 mils angular height or 550 
yards height. The conclusions may be easily modi- 
fied to apply to any height finder using a height- 
conversion mechanism similar to that used in the 
Ml instrument. The following conclusions are made 
on the basis of the theoretical calculations: 

1. Since the angular height of an aerial target is 
determined by pointing the elevation elbow tele- 
scope, the line of sight of this telescope should be 
precisely level when the alignment of the wedges is 
checked. 

2. If the difference between the range-infinity and 
height-infinity settings is not to be readjusted, then 
the resulting errors are minimized (a) by making the 
height-900-yards and range-infinity settings, rather 
than the height-550-yards and height-infinity set- 
tings, coincide when adjusting the height-conversion 
mechanism, and (b) by making the internal adjuster 
setting with the range-height lever in the height 
position and the height finder proper elevated to 
650 mils. 

The following recommendations are made. 

1. The total movement of the fine elevation ad- 
justment shall not be less than 1 degree of the true 
field. 

2. With the horizontal reticle line in the elevation 
elbow telescope falling on an established level line 
and with the tripod and cradle level, fine adjuster 
settings each shall be made at height-infinity, and at 
height-900-yards. The medians of these two sets of 

6J5 


RESTRICTED 


64 


MISCELLANEOUS INSTRUMENT AND OPERATIONAL DEFECTS 


readings shall differ by less than 2.0 UOE. The 
medians at range-infinity and height-900-yards shall 
differ by less than 1.0 UOE. 

3. Set the microscope on the elevation scale to 
read 0 degree when the range-height lever is at the 
height position. When the lever is moved to the 
range position, the elevation angle shall be 90 degrees 
±30 minutes. 

4. When the horizontal reticle line from the eleva- 
tion elbow telescope falls on an established level line, 
the horizontal reticle line in the azimuth elbow 
telescope shall fall within 2 minutes of this line and 
the level bubble on the height finder proper shall be 
within 0.5 division of the center of the scale. 

5. When the horizontal reticle line in the eleva- 
tion elbow telescope falls on an established level line, 
the fine elevation adjustment shall place the center 
lines of the main reticle marks at least 24' above and 
below the established level. It is believed that these 
modifications will ensure more accurate range and 
height readings in the field and also make the speci- 
fications easier for the manufacturer than those then 
existing. The use of the relation of the elevation elbow 
telescope to an established level line, instead of the 
use of the level on the heightfinder proper, to deter- 
mine the correct elevation or depression of the height- 
finder is deemed important. It is also recommended 
that consideration be given to the design of a suitably 
portable instrument permitting the simple and rapid 
establishment of an actual or artificial level line for 
the elevation telescope. 

This last recommendation is considered further 
in a subsequent report of the Princeton Branch of 
the Frankford Arsenal which presents a design of a 
simple optical instrument for accurately leveling the 
line of sight of the elevation tracking telescopes of 
height finders, directors, and optical trackers and 
gives a summary of the instrument’s performance 
when used to collimate the elevation tracking tele- 
scopes of the Ml Height Finder. (257) This instru- 
ment is described in the text and sources of error are 
discussed. To test the level collimator, it was mounted 
on the elevation tracking telescope and a level point 
was provided by sighting the azimuth tracking tele- 
scope on the reticle grid of a transit. Repeated level 
settings were made using the level collimator and 
after each trial the alignment of the horizontal reticle 
mark in the azimuth tracking telescope was checked 
against the fixed reference grid of the transit. The 
reticle graduations of the transit used were such that 


its accuracy as a calibrating instrument was no better 
than 1 minute of arc. All level settings using the 
level collimator were made within the limits of accu- 
racy obtainable with the transit (0.3 mils). Following 
each level setting made with the level collimator 
mounted on the elevation telescope, the alignment 
of the horizontal reticle mark in the azimuth track- 
ing telescope was checked against a grid of known 
dimensions placed at a distance of 60 yards from the 
height finder. By stopping down the objective of the 
azimuth tracking telescope to 0.25-inch aperture to 
eliminate parallax, the inaccuracy of this test tech- 
nique was 0.5 minutes of arc or 0.15 mils. Repeated 
determinations indicated that the height finder could 
be levelled within these limits of accuracy. 

Unequal Light Transmission 

At the Admiralty Research Laboratory experi- 
ments were carried out which demonstrated that 
false stereoscopic effects were produced in stereo- 
scopic systems where a difference in brightness exists 
between the two fields of view when there is relative 
azimuth motion of the target and reticle. (51) In the 
Ml Height Finder, which has light transmissions of 
19.5 and 13.6 per cent for the two telescopic systems, 
the time lag between the eyes of the observer is 1 /400 
second. This has been confirmed experimentally by 
observations with this instrument. Assuming that the 
rate of transverse of the height finder may be in error 
by 10 minutes of arc per second when following mov- 
ing targets, the presence of a time lag of this magni- 
tude would cause an error in range of 1.5 seconds of 
arc, or 3 UOE with a 24-power magnification. 

Lack of Parallelism of Emergent Rays 

A study of the effect of lack of parallelism, in the 
vertical meridian, of the emergent ray in an Ml 
Stereoscopic Range Finder was studied experi- 
mentally by HMS Excellent. (302) The effect was 
produced by placing a prism base up over one eye- 
piece and another equal prism base down over the 
other eyepiece. In this way errors were produced as 
great as 60 minutes of arc of apparent field in the 
height of the two presentations, both reticle and 
target. This is not to be confused with height adjust- 
ment of target and reticle. It was found that even so 


RESTRICTED 


PSYCHOLOGICAL BIAS 


65 


large a lack of parallelism does not produce any 
significant falling off in precision as measured by 
mean consistency (scatter of observations about the 
mean). It was found that there was no falling off of 
consistency under these extreme conditions even 
after the observer had been fatigued by continued 
ranging. It is considered that lack of parallelism of 
emergent rays in this amount fails to cause a falling 
off in consistency because the range taker can partly 
compensate for the discrepancy by tilting his head, 
so long as the tilting is not so great that the eye can- 
not see through the exit pupils of the range finder. 
The conclusion is reached that the manufacturing 
tolerance of dzlS minutes of arc of apparent field is 
adequate. 


in the atmosphere lying between the target and the 
instrument, (2) internal or instrumental errors— all 
errors originating within the instrument, many of 
which can be eliminated or ameliorated by modifi- 
cation of design and construction, (3) physiological 
errors or those which arise from the performance of 
the eye as an optical instrument, and (4) psycho- 
logical errors or all errors that originate beyond the 
retina. Although any and all of these errors may 
seriously affect the accuracy of an individual read- 
ing, it is pointed out that when the range finder is 
used in connection with a director, the readings are 
used to determine the time rate of change of range 
and their effects are somewhat decreased. 


Backlash 

The Applied Psychology Panel report a study of 
backlash between the main bearing race and the 
bevel pinion in the Ml and M2 Height Finders. (69) 
Very little attention had previously been given to 
backlash between the main bearing race and the 
bevel pinion of Ml and M2 Height Finders as a pos- 
sible source of error in height measurements. The 
presence of backlash in this mechanism is of consid- 
erable importance because the present wedge inspec- 
tion procedures and wedge adjustment tolerances 
are based on the assumption that backlash is absent 
between the bevel pinion and the main bearing race 
when the height-range level is put in the height 
position. Three different procedures were used in 
measuring the amount of backlash in eight Ml and 
two M2 Height Finders. The results show that the 
amount of backlash is often very large. Its presence 
results in short height readings on incoming aerial 
courses and long height readings on outgoing aerial 
courses. The report recommends inspection and re- 
duction of such backlash in all instruments. 

8 2 ERROR STRUCTURE OF RANGE 
AND HEIGHT FINDER 

A report from the National Bureau of Standards 
theoretically analyzes the kind of errors which may 
be expected in the operation of a range or height 
finder. (322) These errors may be either of the syste- 
matic or accidental type. They fall into four general 
classes: (1) external errors— those which originate 


*•2*^ Psychological Error 

On the basis of this analysis, the National Bureau 
of Standards reports a quantitative study of one type 
of error of the psychological sort. (323) It has often 
been assumed that readings made with the stereo- 
scopic range or height finder might be affected by a 
psychological bias, different for different observers 
or for the same observer on different targets, which 
causes settings to be inaccurately made and introduces 
range errors that are independent of the instru- 
mental accuracy. The test of a captured German 
range finder (Type R 40), which is provided with 
means for making observations with ortho or with 
pseudo stereoscopic viewing, enables this error to be 
measured. The values obtained are surprisingly large 
and of considerable importance. All tests were made 
on terrestrial targets. The instrumental performance 
of the R 40 Range Finder is excellent, instrumental 
errors amounting to no more than plus or minus 1 
second over ranges varying from 19,000 to 1,400 yards. 
Nevertheless, experienced observers frequently make 
errors as large as 2 seconds and, on one target, an error 
of 5 seconds was made because of psychological bias. 
For this particular instrument, 5 seconds correspond 
to 15 UOE. Psychological errors of this kind may be 
sufficiently systematic to be partially compensated for 
by the uniformity of targets when a height finder is 
used against planes, or for use against naval targets. 
Such compensation does not appear practical for 
terrestrial targets. There are reasons for believing 
that this psychological error may be even greater for 
the Ml Height Finder than for the captured German 
instrument. 


RESTRICTED 




66 


MISCELLANEOUS INSTRUMENT AND OPERATIONAL DEFECTS 


These results lead the author to suggest the follow- 
ing considerations: (1) In estimating the quality of 
performance to be realized in a stereoscopic range 
finder for use in a tank or by the field artillery against 
terrestrial targets, the possibility of an error due to 
psychological bias amounting to 2 or more seconds 
should not be overlooked. (2) The coincidence or 
vernier type range finder should not be abandoned 
too hastily. It is probably less subject to this psycho- 


logical error than is the stereoscopic type. In this 
connection, it should be noted that the development 
work at the National Bureau of Standards is proceed- 
ing along parallel lines with the stereoscopic and 
vernier types of instruments. (3) The ortho-pseudo 
stereoscopic instrument assumes increased impor- 
tance not because of its doubled sensitivity but be- 
cause it is possibly free, or relatively free, from errors 
arising from psychological bias. 



PART II 


THE MAN-INSTRUMENT COMBINATION 


T he four chapters (Chapters 9 to 12) deal with the 
range finder when used by a human operator. It 
has been found that the range finder, under the best 
operating conditions, seldom if ever gives results 
whose accuracy, precision, or consistency closely ap- 
proach the theoretical expectation of the instrument 
itself. These four chapters deal with experiments 
which seek to isolate the causes of such discrepancies 
due to the operator and his manipulation of the 
instrument. 

Chapter 9 deals with certain psycho-physiological 
factors of the operators. The results are largely nega- 
tive because it was found that fatigue, lay-off, loud 
sounds, and sex differences were relatively unimpor- 
tant. Although some of these factors seem to be 
effective, as for example severe fatigue, it was found 
that if the operator could be sufficiently highly 
motivated, he could seemingly lift himself by his boot- 
straps for a short time and give as good ranges as in 
the unfatigued state. Drugs were applied with two 
goals in view. It was found that the administration of 
benzedrine sulphate showed slight advantage in over- 
coming drowsiness and that metrazol, a cerebral and 
respiratory stimulant, had no significant effect on the 
variability of range estimates in normal operators. 

Chapter 10 deals with experiments having to do 
with the relative position of reticle and target. The 


results show that the so-called height-break is of great 
importance for accurate ranging. This is the failure 
to adjust the target properly and the reticle pattern 
correctly. Hence, although in some instruments the 
range finder operator has control of a fine height ad- 
justment system, these studies emphasize the impor- 
tance of tracking so that the target image may be 
kept in the correct position with regard to the reticle. 
Fundamental experiments dealing with the analysis 
of factors for improved tracking mechanisms and 
others studying reticle designs which will reduce the 
effect of this misalignment are summarized. 

Chapter 11 deals with fundamental laboratory 
studies of the effects of haze and of atmospheric 
scattering. Although the results indicate that such 
atmospheric effects are reflected in the accuracy of 
range measurements, it is now considered that these 
are less important, in field practice, than was for- 
merly believed. The experiments show that differ- 
ence in contrast between the target and its back- 
ground is the important variable in this situation. 

Chapter 12, the final chapter in this group, deals 
with miscellaneous factors of operation, such as 
techniques for maintaining stereoscopic contact be- 
tween target and reticle, focussing of the telescopic 
eyepieces, and the use of the range finder as a spot- 
ting instrument. 


67 


4 


.-t 




p . 



i 


\ 

t 




■<*'•■' V* 
-T-^ i .0 ■ 


t 




1 ^- 




!" 


l‘■‘ 


i 


i 


f 


,0 


I 




I 




Jl 


t 


4 t 


4 




'*■ 


■n. 



t 


I 




* 



•V" 




4 


1 


4 


\ 




t 

‘ I 


*» •• 












.1 

* V 


I* 













Chapter 9 

PSYCHO-PHYSIOLOGICAL FACTORS OF OPERATORS 


A SERIES of studies were made to determine the 
effects of certain psychophysical factors of stereo- 
scopic operators upon the accuracy and variability of 
ranging. Many of these results are summarized in a 
Report to the Services issued by the Fire Control Sec- 
tion of NDRC. (12) Some of the material included in 
this report will not be found elsewhere. This chapter 
deals with carefully controlled experiments per- 
formed in a number of research laboratories. 

9 1 SOME BASIC STUDIES 

At the Harvard Fatigue Laboratory it was found 
that the angular limits of binocular fusion increase 
as the angular size of the target increases. Four sub- 
jects made observations with a specially devised 
apparatus. (269) The amounts of the fusion limits 
are of considerable extent, being doubled when the 
target size is increased 16-fold. All of the curves are 
of similar form for all four subjects— starting with 
low fusion limits for small target size, increasing 
rapidly and then more slowly as the target size is 
increased. This result raises interesting questions 
concerning the use of binocular retinal disparities 
to occasion changes in apparent distance by means 
of stereoscopic instruments which purport to meas- 
ure range accurately. For example, as the angular 
size of the target increases and thus, as the angular 
range throughout which disparities are effective is 
increased (1) is each “unit” of apparent distance 
magnified, i.e., multiplied by a constant, or (2) do 
these units remain constant in size while their num- 
ber is increased? 

In a second study at the Harvard Fatigue Labora- 
tory a study was made of the limits of binocular 
fusion as dependent upon binocular vergence. (270) 
It was found that for proximal distances (about I 
meter) and for a given target-reticle assembly, the 
limits of binocular fusion in angular units are found 
to be essentially constant. It was important to dis- 
cover these limits because it is only within such limits 
that binocular disparities can be effective for normal 
stereoscopic vision. A specially constructed appa- 
ratus was employed (cf. 272 for description) and the 
same four subjects were employed. It was found that 
the curves for all subjects were very similar, showing 
an increase of the fusion limits for disparate images 


as a straight line relationship with the converged 
distance. When ocular vergence is fixed, the limits of 
binocular fusion for any given degree of vergence 
appear to define the limits throughout which binoc- 
ular retinal disparities are effective. In this present 
experiment, the target angle subtended a visual 
angle of about 1 minute and the range throughout 
which binocular retinal disparities were effective was 
found to be constant at approximately 10 minutes 
of arc. 

A third Harvard Fatigue Laboratory experiment 
had to do with apparent size and binocular vergence. 
(272) It was found that the apparent visual size of 
a target is an increasing function of the converged 
distance. The rate of change in the apparent size of 
the target with respect to binocular vergence is 
directly proportional to the size of the target. Quali- 
tatively, this effect is quite noticeable in all stereo- 
scopic range finders, inasmuch as when the target 
recedes it seems to become larger, which is exactly 
opposite to the normal effect in unaided stereoscopic 
vision. The figures for the relation of apparent size 
and binocular vergence and for the rate of change of 
apparent size are extremely similar for all four sub- 
jects. 

A fourth laboratory study of this series, performed 
at the Harvard Fatigue Laboratory, is concerned 
with the relation of binocular vergence and target 
size on apparent distance of the target. (271) The 
results were obtained from the four observers previ- 
ously used working with a special apparatus. The 
results are consistent with the notion that the appar- 
ent distance of a target is practically independent of 
its size. From one observer to the next for any given 
target size and vergence, and for any given observer, 
the differences in measurements are randomly or- 
dered with respect to the vergence of the eyes. The 
angular size was varied from 0.5 degrees to 2 degrees, 
a ratio of 4 to 1. The variation from one target size 
to the next for any given degree of vergence lies 
within the limits of variation usually found when a 
single target was used. Hence the investigators con- 
clude that the average apparent distance, as a func- 
tion of the converged distance, or the point in space 
to which the eyes are converged, appears to be inde- 
pendent of the size of the target for the sizes studied. 

At Ohio State University a careful laboratory study 

69 


RESTRICTi 


70 


PSYCHO-PHYSIOLOGICAL FACTORS OF OPERATORS 


was made to determine the dependence of cyclo- 
phoria on eye and head position. (329) It is known 
that cyclophoria may be produced with changes in 
eye and head position such as the operator must 
make to compensate for the turning of the range 
finder for vertical tracking changes. This might 
introduce false differences in binocular parallax 
between the target and reticle when a vertical sep- 
aration exists between them. The reason for the 
appearance of such an error can most readily be seen 
if one considers the situation in which the reticle 
and target are actually in the same stereoscopic 
plane, and one is displaced above the other. For the 
observer with cyclophoria the ocular images of the 
reticle and target will have a different relative lateral 
displacement with respect to the physiological verti- 
cal meridians of his retinas, and this displacement 
will be equivalent to that normally introduced by a 
difference in depth between target and reticle, and 
would be interpreted psychologically as such. Be- 
cause of this practical problem and because of more 
theoretical considerations, a general survey was 
undertaken of the dependence of cyclophoria on 
various positionings of the eyes in the orbits and with 
respect to each other, as well as a preliminary investi- 
gation of the dependence of cyclophoria on the posi- 
tion of the head. 

Records were taken with a synoptoscope, a haplo- 
scope, and a head-positioning instrument, which are 
described in the text. Nine subjects were used in a 
number of experiments attacking different variables 
of the problem. The results indicate, in spite of a 
considerable consistency of the data on cyclophoria 
in relation to eye position, as formulated in terms of 
Listing’s Law, individual variations and discrepan- 
cies made it impossible to predict cyclophoria 
changes in general, at least without data on each 
individual, even under the rigidly controlled labora- 
tory conditions and with subjects trained to make 
these observations. It therefore seems necessary, and 
by all odds easiest, to avoid the situation in the range 
finder by avoiding vertical separation of target and 
reticle which permits changes in cyclophoria to pro- 
duce range errors. Failing this, or as a supplement 
to this attempt, the authors recommend that an effort 
should be made to keep the target well-centered 
laterally on the reticle, since this would obviate 
lateral eye movements, which might induce changes 
in cyclophoria. Both of these recommendations indi- 
cate the need for tracking accuracy. Furthermore, the 


failure to discover significant changes in cyclophoria 
with changes in head position indicates that there is 
no source of error related to this activity in range 
finding. 

An investigation of the interval of time elapsing 
between the making of a range and the signal that 
the range has been made was undertaken at the 
Howe Laboratory of Ophthalmology. (309) A special 
apparatus described in the text was used and three 
trained subjects were employed. The situation was 
a simulated diving target which corresponded to a 
plane diving from 25,000 feet to 5,000 feet at a con- 
stant velocity of 296 knots. Both hand and foot 
depression of a telegraph key were employed. It was 
found that the average interval between making a 
range and report by key tapping of its having been 
made is about 0.08 seconds. Hence it was recom- 
mended that the dead time settings should not be 
altered because of an assumed interval between the 
time when range has been made during continuous 
contact operation and the time when the range finder 
operator signals that the range has been made. No 
significant differences in this time interval were 
found for signal with hand or foot operation. 

92 FATIGUE AND MOTIVATION 

It was suspected that range finding over a long 
period might result in reduced efficiency due to 
fatigue and/or monotony. A series of experiments 
were performed to discover the effects of fatigue or 
of motivation as it might affect either fatigue or the 
monotony of the task. Much of this material is sum- 
marized in a report from Tufts College. (572) For 
most of the fatigue tests the situation was made as 
realistic as possible to simulate a military situation. 

In one experiment at Tufts College the modified 
dynamic Mark 1 1 Navy Trainer was employed. The 
observer was required to track continuously in azi- 
muth an airplane target as it moved in an irregular 
pattern across the field. (549) A single observer per- 
formed the task continuously for IS]/^ hours. The 
subject was paid on an hourly basis and, when a 
decrement in performance appeared, the pay rate was 
increased in an effort to introduce additional incen- 
tives in an attempt to maintain maximum efficiency. 
In an earlier experiment five subjects performed this 
task but without special motivation. 

In the first experiment, it was found that although 
continued performance for 4 hours frequently 


RESIRICTE] 


TUFTS COLLEGE FATIGUE EXPERIMENTS 


71 


showed a marked decrement— as an increase in the 
frequency of large errors— there were great individ- 
ual differences. One observer who tracked for 4 hours 
showed no significant decrement whatever. Further- 
more, the capacity of a trained observer, as measured 
by his accuracy during the first 5-minute period of 
tracking, and his performance during three periods 
of 4 hours, bore little relation to each other. One 
subject, however, whose error score varied from 2 to 
4 in the first 5 minutes, never exceeded an error score 
of 4 for the first 3 hours of his first 4-hour period, 
but never had an error score of less than 7 for the 
first hour and a half of his third 4-hour period. An- 
other subject, whose error score \’aried from 5 to 7 
during the first 5 minutes, had error scores for his 
first 4-hour period which only once were less than 19 
and error scores for the second 4-hour period which 
only once were above 14. A striking feature of the 
decrements induced by prolonged tracking was that 
they could be offset to a considerable degree by the 
introduction of various “incidents” such as rests, 
promised increase in pay, and encouragement in 
making an effort. These improvements, however, 
were of very short duration. The investigators dem- 
onstrated that a decrement in tracking performance 
could be produced by long continued activity in the 
same task and that the decrement may be reduced 
by adequate motivation but not to initial perform- 
ance levels and finally that such temporary improve- 
ment is short lived. Hence they conclude that in the 
case of tasks such as azimuth tracking, which do not 
involve any considerable muscular effort, perform- 
ance decrement comes from boredom rather than 
physiological fatigue, though it may not occur at all, 
or be greatly reduced, if boredom is prevented. 

A second experiment at Tufts College involved a 
3-day test of fatigue effects under conditions of long 
hours of duty and limited sleep. (550) This experi- 
ment was designed to approximate actual field condi- 
tions, where fatigue effects, if they occur, are the 
result of activities which intervene between the brief 
periods of actual operation of ranging instruments. 
Over a 3-day period, a group of four observers was 
required to work, eat, and sleep according to a highly 
irregular schedule, with short periods of sleep and 
long hours of duty. During these periods on duty, 
the men were constantly on the alert and they were 
required to record the time of occurrence and nature 
of certain light flashes which were produced at irreg- 
ular intervals on the viewed terrain and at distances 


of 376 to 945 feet from the observers and in an 
angular field of 30 degrees. The subjects were also 
required to report at intervals to the laboratory for 
short tests of stereoscopic ranging and tracking. Al- 
though by the end of the test it was evident that the 
men had been under strain, a comparison of their 
performance on the various tasks for the first and 
second halves of the 72-hour period showed no 
decrement. Hence the investigators conclude from 
this experiment that decrease of sleep to 16 hours 
out of 72, combined with alertness for 42 hours out 
of 72, need not result either in decreased efficiency 
as a watcher or in decrement of performance in 
stereo ranging and tracking. 

Still another Tufts College experiment had to do 
with the effects of deprivation of sleep for 50 hours 
upon stereoscopic ranging performance. (551) Ten 
observers took part in this experiment and they were 
tested regularly during this period on both ranging 
and tracking performance. It is reported that these 
subjects were exhausted at the end of this 2-day 
period of wakefulness. However, comparisons of 
performance during the first 24 hours and the second 
24 hours in tracking accuracy indicated no decre- 
ment. A similar comparison in regard to stereo range 
determination showed a decrement in precision, but 
the magnitude of the decrement was not great 
enough to be considered a serious factor in affecting 
accuracy of fire. Certainly the differences were not 
statistically significant for anyone of the 10 observers. 

These subjects were also given certain non-military 
tasks during this period, such as arithmetic, reading 
rate and comprehension, and steadiness tests. No 
decrements were found in the scores of any of these 
measures. On the other hand, from clinical observa- 
tion of the subjects and from the subjects’ own re- 
ports, there is no question but that the group was 
“tired” in the ordinary sense of the term, while not 
irritable, the observers became increasingly less 
social, and greatly less responsive. Their conversation 
was often nonsensical, incoherent, and irrelevant; 
they grew progressively indifferent to their surround- 
ings, eventually showing a definitely lessened inter- 
est in all social activity. They kept their performance 
records from falling as the experiment progressed by 
making a greater effort to attend to the tasks. 

Still another experiment at Tufts College was con- 
cerned with the effects of prolonged strenuous exer- 
cise. (553) Two groups of five men were made to 
march 30 miles, an activity to which they were not 


fRESTRICTED ' 




72 


PSYCHO-PHYSIOLOGICAL FACTORS OF OPERATORS 


accustomed. Their performance in a number of 
tasks was tested before and after the march, and also 
at the end of the first and second 10 miles. The tasks 
involved were stereoscopic acuity and tracking ac- 
curacy, brightness discrimination, reaction times, 
and persistence at a routine clerical task. Special 
monetary incentives were used to keep effort at a 
high level. Although the observers showed unmis- 
takable clinical signs of fatigue, such as sleepiness, 
slouching, garrulousness and silence, no decrement 
appeared in their stereoscopic acuity, tracking ac- 
curacy, sensitivity to brightness differences and re- 
action times. The test of persistence, however, which 
lasted an hour and a half, did show a statistically 
significant decrement. Hence a decrement in some 
performance was demonstrated as a result of the 
fatigue from the hiking activity. However, prolonged 
strenuous exercise does not seem to affect perform- 
ance in tasks such as ranging and tracking, provided 
the duration of these tasks is short and a strong in- 
centive is present. Such fatigue, however, does affect 
adversely persistence at a task over a period as long 
as 11/2 hours. 

Tufts College performed another experiment in 
this series which had to do with the effects of short- 
period exercise on stereoscopic ranging. (554) In this 
experiment, nine well-trained observers were sub- 
jected to a period of rapid stair climbing carrying a 
very heavy load. Immediately following the exercise, 
they were given a test of stereoscopic range finding, 
the results of which could be compared with similar 
tests taken just before the exercise period. The exer- 
cise consisted of three trips up and down a 30-foot 
staircase carrying a 36-pound weight. This was per- 
formed rapidly, as the entire exercise lasted only 
from 87 to 110 seconds. Observation of the subjects’ 
reactions indicated that the effort was extreme. They 
showed the common symptoms of physical strain 
and breathlessness, and several remarked that they 
would be unable to make another round trip. Com- 
parison of the standard deviations of the ranging 
sittings showed that a statistically significant increase 
in variability was present after the exercise. The 
average amount of this increase was about 0.5 UOE. 
Analysis of the results by groups of ten settings 
showed, however, that the increase in variability was 
very short lived, disappearing by the end of 5 minutes. 

There is evidence that this decrement is not the 
result of fatigue but of hyperventilation due to 
breathlessness. An experiment at the Harvard Fa- 


tigue Laboratory on this problem was performed by 
having the subjects breathe deeply to the rhythm of 
a metronome set at 32 or 50 cycles per minute. (279) 
The subject maintained this rhythm until he ex- 
perienced spells of dizziness or a blurring of the 
visual field. Immediately following this, the subject 
made 10 range estimates using either stereo or vernier 
acuity. The results show a marked and consistent 
increase in variability which occurs immediately 
after the hyperventilation. Without exception, the 
subjects made poorer as well as more variable judg- 
ments following hyperventilation. Observers some- 
times had difficulty in fusing the reticles for 30 to 
45 seconds after hyperventilation. The effects were 
more pronounced on stereoscopic acuity than they 
were on vernier acuity. 

Another Harvard Fatigue Laboratory experiment 
had to do with the effects of exercise but without 
breathlessness. (275) Both stereoscopic and vernier 
acuities were tested. The exercise consisted in pedal- 
ing a bicycle ergometer adjusted for an 8-pound pull 
for 1 mile at top speed. Under these conditions of 
rather strenuous exercise, no effect was discovered 
on either stereoscopic or vernier acuity. 

In a final experiment in the series of Tufts College 
there was studied the performance of trained sub- 
jects on a complex task of 4 hours’ duration. (573) 
The observer was instructed to operate continuously 
a knob regulating the rotation of speed of an inner 
dial, so as to align a pointer with an outer dial whose 
irregular speed of rotation was governed by a cam 
control. A number of accessory tasks were made to 
occur in a random sequence. He was required to 
watch a clock and indicate when each 10-minute 
period had elapsed; to signal when a model airplane 
reached certain points on a map; to indicate the pres- 
ence of another aircraft which appeared on any of 
the four quadrants of the map. Five high school boys 
were used as subjects and they were given a week of 
preliminary training before the recorded trials were 
begun. This experiment was planned to test the 
hypothesis that observers would show signs of fatigue, 
as indicated in a decrement in efficiency of perform- 
ance, when continuously engaged for relatively long 
periods of time in a task of psychological complexity. 
The results indicate no significant changes in per- 
formance were observed over the 4-hour period. 
From these results the authors conclude that ob- 
servers may work continuously and effectively at a 
psychologically complex task of this type for several 


RESTRICTED 



DARTMOUTH EYE INSTITUTE EXPERIMENTS 


73 


hours without any significant change in the level of 
performance. 

There was some clinical evidence which indicated 
that small degrees of aniseikonia, which were nor- 
mally compensated for by the subject, might be so 
enhanced by fatigue that this compensatory control 
might be lost by the individual and the effects of false 
spatial localization resulting from the defect might 
become operative. A series of experiments were per- 
formed at the Dartmouth Eye Institute to study this 
problem. (189) The Dartmouth Tilting Board, 
described in the text, was employed. Subjects were 
fatigued by a continuation of normal activities over 
periods from 24 to 40 hours. The space eikonometer 
was also employed. In the first experiment, both 
normal and fatigue data were obtained from 1 1 sub- 
jects. With one exception the subjects showed no 
effects on spatial localization resulting from fatigue. 

In a second report the tilting field was somewhat 
modified. (190) In this way the two parts of the 
field provided disparity clues for purposes of orienta- 
tion. One part also provided a definite form clue. 
A total of 29 subjects were tested with this new device. 
Size lenses were worn in part of the experiment to 
produce aniseikonic effects. Under certain conditions 
there seemed to be a trend for some subjects to use 
stereoscopic and others to fall back on monocular 
clues such as form and perspective under the fatigue 
condition. It was also found with the space eikon- 
ometer that variations in stereoscopic response occur 
from day to day and under different conditions of the 
individual. All subjects when fatigued reported diffi- 
culty in making settings with the space eikonometer. 
This difficulty varied from a subjective uncertainty 
as to the exactness of their judgment to reports of 
variations in the appearance of the targets them- 
selves. The time consumed in making the space 
eikonometer measurements was always longer when 
the subject was presumably in a fatigued condition. 
The interesting and paradoxical situation occurred, 
however, that in spite of this subjective difficulty 
and uncertainty, the actual measurements were made 
with the same degree of accuracy as under normal, 
unfatigued conditions. 

A third report from the Dartmouth Eye Institute 
outlines experiments on 28 subjects with the modi- 
fied tilting board. (191) Marked differences in 
binocular response are found among different indi- 
viduals and in the same individual under different 
conditions, among which are those presumably re- 


sulting from fatigue. An outline of the problems to 
be investigated is included. The quantitative data 
for this report will be found elsewhere. (192) 

A fourth report from the Dartmouth Eye Institute 
records further experiments on the effect of fatigue 
on spatial localization. (193) The apparatus was 
modified so that the effect of form clues upon the 
responses to specific types of binocular disparity re- 
lationships could be investigated. Eor example, if 
the effect of form upon localization based on hori- 
zontal disparity clues was to be studied, the stimulus 
situation (i.e., the fixed targets) should contain no 
clues that could be responded to on the basis of dis- 
parity relationships other than horizontal disparity 
differences. Hence a vertical tipping board was de- 
vised for these further experiments, as well as a modi- 
fication of the space eikonometer. No new results of 
importance were reported. These experiments from 
the Dartmouth Eye Institute are summarized in a 
final summary report. (194) So far as effects of fatigue 
upon spatial localization are concerned, the general 
conclusion is that when subjects have gone without 
sleep for at least 24 hours, marked variations, in re- 
spect to accuracy and consistency with which they 
respond to introduced differences in binocular dis- 
parities, occur for some subjects. These variations 
however, cannot be considered to be consistently or 
characteristically different from the usual day to day 
variations. Erom this the investigators conclude that 
the experiments yielded equivocal results with re- 
spect to the effect of fatigue on stereoscopic spatial 
localization. Certainly these experiments failed to 
demonstrate any consistent fatigue effect. 

As a result of these experiments at Tufts College 
and the Dartmouth Eye Institute, the Fire Control 
Division of NDRC made a report to the Services. (29) 
None of the tests at either institution showed im- 
portant degrading effects. Indeed the results indi- 
cate that the ocular functions employed both in 
stereoscopic ranging or tracking are extremely re- 
sistant to fatigue effects. This is certainly true for 
relatively short periods of operation following either 
loss of sleep or loss of sleep plus long periods of alert- 
ness or exercise. However, it has been demonstrated 
that the power of persistent attention over a long 
period may be affected by such a procedure. This 
conclusion is true, although the subjects may show 
significant clinical signs of general fatigue as indi- 
cated by marked personality changes. Hence it was 
concluded that no special precautions against fatigue 


RESTRICTED ' 



74 


PSYCHO-PHYSIOLOGICAL FACTORS OF OPERATORS 


are necessary in the case of tracking or stereoscopic 
ranging personnel. 

^•2-^ Some Remarks on Motivation 

One by-product of the experiments on fatigue, 
noted above, is that increased motivation may tem- 
porarily reduce the decrement of performance pro- 
duced by the fatigue condition. There are several 
other experiments reported by the Tufts College 
group which bear upon this topic. The first study 
has to do with the effects of knowledge of results dur- 
ing training in ranging on a moving target. (563) 
Six observers were trained on stereoscopic range 
finding on the Tufts Trainer, in which the operator 
is required to adjust the distance of a moving target 
so that it will be at the same distance as a reticle 
mark. Knowledge of results was given the observers 
by sounding a buzzer whenever the distance between 
target and reticle exceeded a predetermined maxi- 
mum, which in some tests was 5 UOE and in others 
2.5 UOE. For four observers, training consisted of 
three tests, each consisting of 30 90-second periods 
without knowledge, followed by one test with the 
buzzer signal sounding whenever the error exceeded 
5 UOE and two tests with signal sounding whenever 
the error exceeded 2.5 UOE. For two subjects the 
buzzer signals, which indicated when the maximum 
allowable error was exceeded, were used throughout 
—for 0.5 UOE in the first test and for 2.5 UOE for 
the other 5 tests. It was found that four of the six 
subjects showed improvement, as measured by a de- 
crease in the frequency of the buzzer signals from 
tests 5 to 6. The two subjects who did not show im- 
provement in these terminal training series were the 
best subject (who was so good that further improve- 
ment should not be expected anyway) and the worst 
subject (whose terminal performance was 24 times as 
bad as that of the best operator and who therefore 
was undoubtedly a person not qualified for this sort 
of training in any case). 

The permanence of the improvement induced by 
knowledge of results was investigated by giving the 
subjects two additional tests without use of the 
knowledge of results signals. The results show that 
the improvement induced by knowledge of results 
began to wear off almost immediately. From these 
results the investigators conclude that the perform- 
ance in tasks such as range finding is improved by 


arranging the situation so that the observer knows 
when he is doing well and when he is doing badly. 
Also, if improvement is to be maintained, knowledge 
of results must be maintained as well. In the range 
finding operation, knowledge of results seems to act 
as an incentive to action, or as a motivating device, 
rather than as a cue to learning. It may be pointed 
out that one of the great advantages of the M6 
Trainer over the M2 instrument is the fact that the 
former has incorporated a recording device and an 
integrator so that the operator under training may 
have immediate knowledge of his results after each 
run. These facts are also incorporated in the Train- 
ing Manual (68,70,71). 

Tufts College investigated another motivational 
device which consisted of bell pacing in the range 
finder operation. (559) The Tufts Trainer was again 
used. Two groups of subjects were used: The first 
consisted of five high school students who made six 
5-minute runs per session for six sessions. For this 
group, a bell sounded every 10 seconds and they sig- 
nalled contact as soon afterwards as they were sure 
they had true range. Group II consisted of six college 
students who made 30 runs of 90 seconds duration 
at each of 15 sessions. For this second group, no bell 
was used in some sessions, whereas in others the time 
interval between bells was either 10, 5, or 3.3 seconds. 
Every subject in both groups was instructed to main- 
tain true range at all times regardless of other varia- 
tions in procedure. A graphic record of results was 
obtained for all runs. The results were somewhat 
equivocal. There was no reliable difference between 
the mean errors under the two conditions, but for all 
subjects consistency was greater (i.e., the standard 
deviation was smaller) for settings at time of contact 
than for settings made at time of onset of the bell. 
However, marked individual differences were dis- 
covered. Thus one subject always reduced his con- 
stant error at bell contact while another subject did 
so only twice in the entire experiment. It was hoped 
that the presence of the bell might act as a motivating 
agent to induce the operator to improve or correct 
his setting. This seemed effective in the case of some 
subjects but not in the case of others. Also it was 
found that the spacing of the bell signals had no 
significant effect within the limits tested. Hence the 
investigators conclude that for ranging which is per- 
formed continuously performance is significantly 
improved in some subjects (and not at all degraded 
for the others) at intermittent times of heightened 


^llESTRlCTEn'l^ 


EFFECT OF LAYOFF 


75 


attention and that one means of obtaining this is by 
some bell pacing device at which times the operator 
is instructed to make a special effort to correct his 
setting. The frequency of the periods of such height- 
ened attention seems to be unimportant for frequen- 
cies from 1 to 3 times per 10-second interval. 

93 EFFECT OF LAYOFF 

In a number of laboratory experiments, interrup- 
tion of testing after subjects had been highly trained 
made it possible to study the effects of a layoff of 
variable length upon range finding performance. It 
had been reported by Service personnel that there is 
a decrease in ranging efficiency following even so 
short a period as a week-end layoff. This is consistent 
with the experience of industry where it is found 
that the worker’s efficiency is relatively low at the 
beginning of a work week and increases toward the 
middle of the week. However, with the transporta- 
tion of Service personnel to remote battle areas, it is 
conceived that considerable periods of time may be 
expected during which the personnel may be unable 
to practice operation of the stereo range finder in- 
struments. 

At the Tufts College laboratory, six subjects were 
given seven test sessions during which they were re- 
quired to range continuously on a moving target on 
the Tufts Trainer. (571) Each test session consisted 
of 30 90-second runs with a 30-second rest period 
between runs. The experimental conditions during 
each test session were not constant inasmuch as this 
was a by-product of an experiment aimed at testing 
another variable. Some of these subjects were given 
knowledge of results training and a motivating bell 
signal and others merely kept continuous contact. 
During this training period, the learning curve 
leveled off to an approximately constant value in 
terms of consistency of performance. After the sev- 
enth training period a 2-week period of no work was 
introduced and the subjects were brought back to 
the apparatus after this time of layoff. The subjects 
were unaware of the fact that a special experiment 
was done on the effects of disuse. This is important 
because numerous psychological experiments have 
shown that the subject will often set or prepare him- 
self for the time span between test and retest if he 
knows what this interval will be. Five of the six 
subjects showed a statistically significant decrement 
in ranging consistency resulting from the 2-weeks 


period of no practice. A special explanation seems 
warranted to account for this single subject whose 
results do not show a decrement. The Tufts College 
investigators conclude from these experiments that 
practice periods in ranging should occur at less than 
2-week intervals and that morale may also be affected 
and motivation lowered by the period of disuse. 

Another experiment on the effects of disuse on 
ranging efficiency was performed at the Howe Lab- 
oratory of Ophthalmology with very much longer 
periods of disuse, of approximately 3 to 6 months 
after the original training. (311) A simplified multi- 
course trainer was employed. Three types of target 
were used: (1) black side lines, (2) a black silhouette 
of a diving plane, and (3) a projected colored motion 
picture of a diving plane. Both fixed and dynamic 
targets were used. The dynamic targets produced the 
rate of disparateness which would be produced in 
a Mark 42 Range Finder by a plane diving at 300 
knots from 25,000 to 5,000 feet with either 0, 100, or 
250 knots regeneration supplied. Five stationary set- 
tings of each type were recorded for each observer; 
while three front-to-back, make-and-break, and three 
continuous tracking runs were recorded for each type 
of target. 

These subjects had had a very considerable 
amount of training on other ranging apparatus be- 
fore the layoff. They had not previously operated 
the simplified stereoscopic trainer used in the final 
tests. But the experimenters are of the opinion that 
the tasks before training are comparable to those per- 
formed after lack of practice. Four of the five subjects 
showed a decrement due to disuse ranging from 7 to 
41 per cent of the previously trained performance 
level. One of the subjects, however, showed a decided 
gain of over 30 per cent following the period of no 
practice. This subject was also one of the experi- 
menters and it may be assumed that his motivation 
remained extremely high for the retest. 

On the other hand, a comparison of the results 
of these six previously highly trained observers with 
those of 21 untrained observers indicates that a 
period of disuse of from 3 to 6 months is by no means 
the basis for the loss of all of the effects of previous 
training. Even after the layoff, the previously trained 
subjects have an error very considerably less than 
those who were beginning training. Hence these re- 
sults show a startlingly great superiority of the previ- 
ously trained observers in spite of the decrement of 
their efficiency due to disuse over a long period. Even 


RESTRICT^ 


76 


PSYCHO-PHYSIOLOGICAL FACTORS OF OPERATORS 


for the fixed targets, which are relatively easy, they 
show a superiority of more than 90 per cent. For the 
more difficult moving target situation, the superi- 
ority of the previously trained observers is very much 
greater— being 139 and 161 per cent for the different 
types of target, and 263 per cent for the most diffi- 
cult situation of the projected motion picture of the 
diving target. It should again be noted that these six 
observers were very highly trained at the start of the 
period of disuse. 

A small amount of data is available from Fort 
Monroe on this problem of disuse in the actual field 
situation. These are results from the Height Finder 
School and were contained in an unpublished mem- 
orandum. These observers were not highly trained. 
The results are given in UOE error for both fixed and 
aerial targets, using the Ml Height Finder, the M2 
Trainer in its dynamic form and the Eastman Train- 
er. Performance at the end of the previous week and 
of Monday and Tuesday performances are compared. 
In all six cases for the three instruments and for 
fixed and aerial targets, these average differences are 
small, and in no case are they statistically significant. 
The variations are of a chance character and show no 
constant trend for the different experimental vari- 
ables. Hence it may be concluded that a week-end 
layoff during training causes no serious decrement in 
ranging efficiency. 

There are no data regarding the rapidity of re- 
learning in those cases where a decrement due to dis- 
use is evident. The entire pattern of results, however, 
is totally in conformity with well-known psychologi- 
cal principles of learning and forgetting and, follow- 
ing these, it can be safely predicted that it would take 
a shorter period of relearning for the operators to 
reach their previous level of efficiency than was re- 
quired to produce a similar increment of improve- 
ment in the initial learning periods. 

The experimental data reported in this section on 
the effects of disuse is somewhat meager. The follow- 
ing conclusions to disuse on stereoscopic ranging 
efficiency seem justified, however, especially as they 
fit into well-known psychological observations for 
somewhat similar operations. (1) A period of disuse 
of 2 weeks or more leads to a decrement of perform- 
ance which is statistically significant for many ob- 
servers. (2) A period of disuse for a single day does 
not demonstrate such a decrement. (3) The amount 
of decrement is affected by such factors as (a) the 
length of the period of disuse; (b) the difficulty of 


the ranging problem; (c) the level of training before 
the layoff and (d) the motivation of the observer. 
(4) The longer the period of disuse and the more 
difficult the problem the greater the decrement. (5) 
The higher the level of training before the period 
of disuse and the greater the motivation of the ob- 
server, the smaller will be the decrement. (6) If the 
operator has been very highly trained before the 
period of disuse, he shows a very great superiority 
over an untrained subject even after a layoff of from 
3 to 6 months, indicating that much of the efficiency 
due to the previous training has been retained in 
spite of the long period of disuse. 

It can be recommended: (1) That all stereoscopic 
range finder operators be as highly trained as the 
situation permits in the initial training period. (2) 
That this training be continued as frequently and 
as intensively as the situation permits. (3) That, if an 
observer has not made stereoscopic range operations 
over a period of a week, he be given an intensive 
period of relearning at the first possible opportunity 
and he be given practice in observing regularly 
thereafter as the situation may permit. 

94 the EFFECT OF LOUD SOUNDS ON 
STEREOSCOPIC RANGING AND 
TRACKING 

It may be believed that loud sounds such as one 
might encounter in battle may have an effect on 
ranging and tracking efficiency. An opportunity to 
test this hypothesis was offered through the coopera- 
tion of Section C-5, NDRC, which made available 
the apparatus and records developed under contract 
with Stevens Institute. This includes a record player, 
amplifier, and specially designed head phones. There 
are separate volume controls, one operated by the 
observer and the other by the experimenter. The 
sounds used are designated as BJ cuts numbers 1 to 7. 
A commercial record (Spe-D-Q7921A, Warfare No. 
15) of air raid sounds, including planes and bombs, 
was also used. The volume was never less than 100 db 
in the experiments and went as high as 128-130 db. 

One experiment was performed at Tufts College. 
(548) The sounds were introduced as a part of a 
fatigue test in a tracking experiment. The subjects 
viewed a target in a Navy Mark II Trainer which 
was displaced in azimuth by a motor-driven device. 
Five-minute samples were recorded at 1 / 2 -hour 
periods throughout 4-hour tests of continuous track- 


RESTRTCTED 


a 


SEX DIFFERENCES 


77 


ing. The sounds were delivered to the subjects in 
three different patterns: (1) A 3-minute unan- 
nounced stimulation at the end of the 4-hour period; 
(2) Twelve sound periods of 2 minutes each during 
the 4-hour period; (3) Sounds introduced for 3 min- 
utes at the middle and end of the 4-hour tracking 
period. In this last pattern, the sounds used were 
those reported as the most unpleasant for each ob- 
server. 

When the sound was introduced at the end of the 
4-hour test (pattern 1), no disruption of perform- 
ance was produced. Indeed, there was improved 
tracking performance in the case of two observers 
and some deterioration in the case of a third. When 
the sounds were introduced periodically during the 
4-hour period (pattern 2) there was found to be im- 
proved performance as measured by the work sam- 
ples taken just preceding the introduction of the 
sound and those taken at p 2 ’hour periods during the 
test. In the final test (pattern 3), the sound in all 
cases improved performance in spite of the fact that 
it was definitely more difficult to endure than it had 
been in the previous trials because the amplitude 
was increased from 120 to 130 db and the duration 
from 2 to 3 minutes. 

Another experiment was carried on at Brown Uni- 
versity. (133) Twelve observers were employed on 
the Brown Stereoscopic Trainer to determine the 
extent to which the presence of loud sounds affected 
an observer’s efficiency in a task consisting of stereo- 
scopic pursuit in range finding. Twelve successive 
runs were given the trained observers on each of 8 
experimental days. On each day a total of 3 minutes 
exposure was given each man to the intense sound 
stimulation ranging from 100 to 128 db and the other 
runs were made without sound for purposes of com- 
parison. The level of sound intensity was set by the 
subject himself before each day’s experiment and 
represents the highest intensity which the individual, 
with some urging, was willing to endure. 

The results indicate: (1) That in the experiment 
as a whole, there is no significant difference between 
scores for the observations with and without stimula- 
tion by intense sounds. (2) That in spite of marked 
individual differences in performance ability among 
the observers, there are no significant individual 
departures from the general finding that the presence 
of intense sounds was without effect on the stereo- 
scopic scores. (3) That the results on the first experi- 
mental day, when the sounds might have caused 


more apprehension and surprise, are not significant- 
ly different from those on the other days. (4) That 
the constant error scores were no more changed than 
were the average deviation scores by the presence 
of loud sounds. These results were obtained even 
though the reports of the subjects indicated that the 
sounds produced effects on the sense of balance and 
of a tendency to pain and even nausea. 

From these results it seems safe to say that loud 
sounds perse do not have an effect on these operators 
affecting efficiency of either stereoscopic ranging or 
tracking. However, it might be that through associa- 
tive processes or the like, such sounds would cause 
difficulties in battle to certain emotionally unstable 
persons. This experiment should be of value as indi- 
cating that the problem of selection would not be in 
relation to sound, but rather to temperament. 

These results are summarized in a Report to the 
Services issued by the Fire Control Section. (15) In 
this report it is indicated that, although muscular 
tension was produced, the introduction of the sounds 
were a relief from the monotony of the tasks, an aid 
in staying awake and hence acted, in some cases, as a 
motivational factor. 

95 SEX DIFFERENCES 

Experiments carried on in several research labora- 
tories utilized both men and women subjects. Hence 
a comparison of sex differences was possible in some 
cases as a by-product of these experiments. According 
to some reports, the British were utilizing women for 
operating tracking and ranging mechanisms on range 
finders at relatively fixed antiaircraft installations. 
It was believed that the policy of both American 
Services of utilization of women by enlistment might 
lead to a similar employment of women’s services in 
the future of this country. 

At Tufts College the performance of a group of 56 
men and of 32 women was measured on the Navy 
Stereo-Trainer Mark 2 and on the Tufts Stereo- 
trainer. (567) Eight scores are available for each sub- 
ject from each of two testing sessions. The men were 
all freshmen enrolled in the Naval ROTC Unit. All 
of these subjects had passed the rigorous visual acuity 
tests required of ROTC candidates, with at least 
20/20 vision in each eye. The women were also stu- 
dents at Tufts College who volunteered to take these 
tests. While this group of women was certainly more 
highly selected than the average of the general popu- 


RESTRICTED 




78 


PSYCHO-PHYSIOLOGICAL FACTORS OF OPERATORS 


lation, it seems probable that they were not as highly 
selected as the male subjects in this experiment, 
certainly with respect to ocular capacity and general 
health status. 

In every case, the eight scores were obtained in a 
single session and repeated in a second session. The 
order of taking the tests in the session was random- 
ized. The interval between the two test sessions 
varied from 2 days to 2 weeks. Each subject was tested 
twice with the two instruments. With both training 
instruments, a single test session included 20 settings 
with the target stationary and two 90-second runs 
with a moving target. The subjects were instructed 
to maintain contact continuously. All subjects were 
untrained at the beginning of the experiment. The 
results indicate that the performance between first 
and second tests was not significantly different either 
in the case of the men or women. In comparing the 
subjects of the two sexes, no differences in level of 
performance could be found by a statistical treat- 
ment of the results for either the initial performance 
in the first test, or on the second test after the small 
amount of practice. 

In another experiment at Ohio State University 
both men and women were employed as subjects in 
connection with a study of the effects of the blurred- 
ness of the target in stereoscopic range finding. (388) 
An apparatus was employed by means of which the 
blurredness of the target outline and the degree of 
contrast between target and background could be 
varied independently. Nineteen men and eleven 
women acted as subjects. Their previous training 
varied from highly trained to relatively untrained. 
The results indicate that there is no sex difference 
which is statistically valid. 

An incidental study of sex differences in aided 
tracking was made at Iowa State University. (317) 
There were no sex differences apparent, the men 
averaging 0.168 mil error and the women 0.171 mil 
error. This was true even though the men had had 
more tracking experience and had undergone more 
selection, on the basis of the tests themselves, than 
had the women. 

At the Foxboro Company, an experiment was per- 
formed with direct tracking by hand wheel. (215, 
2 1 6) The presentation was simplified as much as possi- 
ble. The subjects consisted of ten men and ten women 
chosen at random from the personnel of the Foxboro 
Company. Without preliminary tests it was not 
possible to obtain completely matched groups of 


men and women, so it was decided to obtain a fair 
sampling of the population of the plant within the 
age limits of 18 to 35 years and also to match groups 
with respect to type of work and educational status. 
The preliminary results indicated a superiority of 
the men’s group in this operation as judged by error 
scores and by variability measures. However, when 
the experiment was carried further, these differences 
were no longer significant after the ninth hour of 
practice. Hence the investigators conclude that, on 
this rather difficult problem requiring a high degree 
of application and dexterity, women learn more 
slowly than men, requiring a larger amount of prac- 
tice to reach the level of men during all portions of 
the learning curve. With increased practice, how- 
ever, the women can attain any level of accuracy 
reached by the men. 

These reports are summarized in a Report to the 
Services issued by the Fire Control Division of 
NDRC. (28) It is here concluded that no important 
sex differences in ability to perform the tasks of either 
stereoscopic range finding or visual tracking have 
been demonstrated, except for the longer training 
period required for women as demonstrated in the 
Foxboro experiment reported above. 

9 6 DRUGS AND OTHER FACTORS 

Benzedrine Sulphate 

Benzedrine sulphate has been suggested as a means 
of temporarily overcoming fatigue and the effects of 
loss of sleep. In order to test the efficiency of this drug, 
experiments were carried out at the Harvard Fatigue 
Laboratory. (273) Six subjects were used in the ex- 
periment with loss of sleep over a period of 23 to 27 
hours. Readings were taken in the evening and again 
in the morning following the sleepless night. Their 
results were compared with those of a second experi- 
ment which was similar to the first except that 20 mg 
of benzedrine sulphate was ingested after the series 
of readings taken in the morning following the all- 
night vigil without rest. Twenty minutes after the 
ingestion of benzedrine sulphate another series of 
range estimates were taken. It was found that loss 
of sleep of a night’s duration resulted in slight, but 
statistically insignificant, increases in the variability 
of range estimates. Five of the six subjects showed 
improved performance after the ingestion of benze- 
drine sulphate. 

As a result of these experiments, the Fire Control 


DRUGS AND OTHER FACTORS 


79 


Division of NDRC made a recommendation to the 
Services as follows. (22) If benzedrine is given to 
overcome drowsiness following long periods of neces- 
sary wakefulness, it should be given with caution 
and, if at all possible, under the direction of a phy- 
sician. The usefulness of this drug is seriously di- 
minished because of (1) the latent time of the effect 
after ingestion, (2) the duration of the effect of a 
single dosage, (3) possible individual idiosyncrasies 
of the effect, (4) a possible deleterious result after 
the effect has worn off, and (5) possible deleterious 
effects of too frequent or too continued dosage. 

Metrazol 

From an entirely different point of view, an experi- 
ment was carried on at the Harvard Fatigue Labora- 
tory to determine if it might be possible to find a 
drug which would improve stereoscopic and/or 
vernier acuity. (274) After conference with physiolo- 
gists, it was decided to try the effects of metrazol (a 
synthetic organic tetrazol compound) because it was 
advised that this drug was the best available cerebral 
and respiratory stimulant and hence was most suited 
for this purpose. Fifty-two experiments were carried 
out on the effects of metrazol on visual acuity. Forty 
of these experiments were made on stereoscopic 
acuity and the remaining 12 on vernier acuity. Metra- 
zol was administered in 1.5 to 7.5 grain dosages. It 
was found that the ingestion of this drug had no 
significant effect on the variability of stereo or 
vernier range estimates. The increase or decrease in 
the time necessary to make 40 readings was calcu- 
lated for stereo acuity. In 100 out of 132 comparisons 
of time in making the judgments, there was an in- 
crease in the speed of range settings after the inges- 
tion of metrazol without an apparent decrease in the 
precision of these settings. Only 5 cases showed a 
decrease in speed after metrazol ingestion. The 
average correlation between the mean variation and 
the time for making judgments was -)-0.50. This 
indicates that metrazol, a respiratory and cerebral 
stimulant, gave rise to both an increase in speed and 
precision of range estimates. No direct relation was 
found between the amount of metrazol administered 
and the amount of increase in speed of judging ob- 
server. The results for vernier acuity showed no such 
effects after ingestion of metrazol. The critical ratios 
for differences between vernier readings before and 
after metrazol were mostly not significant so far as 
variability of readings was concerned. In spite of the 


slight advantage in speed of making stereo judgments 
which were demonstrated by this study, it was not 
deemed that sufficient improvement in stereo ranging 
had been demonstrated and hence the Fire Control 
Division of NDRC made no recommendation to the 
Services advising the use of this drug. 

Startle 

Still another experiment carried on at the Harvard 
Fatigue Laboratory had to do with the effects of 
startle on pupil size and its effects on stereo and ver- 
nier acuity. (276) It is well known that one of the 
effects of sudden startle, such as would be produced 
by the unexpected firing of a pistol just behind the 
subject, is a dilation of the size of the pupil of the eye. 
Photographs of the pupil taken before and after the 
startle demonstrated an increase in pupil size of an 
average of 0.3 mm immediately after stimulation. 
The pupil size gradually returns to normal during 
the course of 1 minute. Although this pattern was 
exhibited almost without exception by all 36 sub- 
jects, it was found that emotionally unstable subjects 
showed a relatively greater increase in the size of the 
pupil. It appeared, however, that this amount of 
variation in pupil size had no signihcance for range 
finding, since there was no change in variability for 
either stereoscopic or vernier acuity. 

Variations in Blood Sugar 

At the Harvard Fatigue Laboratory a study was 
made on the effects of variations in blood sugar on 
stereoscopic and vernier acuity. (277) Sixteen sub- 
jects came to the laboratory in a basal condition and 
observations were made on stereo acuity. They were 
then given dextrose to observe any possible effects 
due to an increase in the blood sugar level. Another 
series of experiments were carried out in which range 
estimates for both stereo and vernier acuity were 
made while the subject was in a basal state and again 
at standard intervals after the injection of insulin in 
order to reduce the blood sugar level below normal. 
At the conclusion of each insulin test the subject 
ingested dextrose, after which observations were re- 
peated on stereo and vernier acuity. These latter 
experiments were twice repeated. In the first series, 
a moderate dose of insulin was given and, in the 
second series, a fairly large dose was given to lower 
the blood sugar to approximately 50 mg per cent. 
It was found that no significant differences were 
observed in the stereo range settings made under 


RESTRICTED 


PSYCHO-PHYSIOLOGICAL FACTORS OF OPERATORS 


basal as contrasted with non-basal conditions. When 
the level of the blood sugar was lowered by insulin, 
however, there was an increase in variability and, in 
some cases, even a departure from true range. Vernier 
acuity appeared to be more easily influenced by the 
lowered blood sugar than stereo acuity. This was 
especially true with the larger doses of insulin. The 
results indicate, therefore, that vernier acuity is more 
easily varied than stereo acuity by the stress imposed 
by lowered blood sugar. Inasmuch as no effect of 
lowered blood sugar was demonstrated for a degree 
likely to be met in actual operations, no specific 
recommendation was made by the Fire Control Di- 
vision of NDRC. 

Low Oxygen and Low Illumination 

The effects of low oxygen and low illumination on 
stereo and vernier acuity were reported by the Har- 
vard Fatigue Laboratory. (278) The effect of reduced 
percentages of oxygen in the inspired air was studied 
in 22 experiments using six subjects. A control ex- 
periment was made with normal air, another series 
with 12 per cent oxygen (corresponding to an alti- 
tude of 10,000 feet), and a third with 10 per cent 
oxygen (corresponding to an altitude of 19,000 feet). 
In the final series of tests the illumination was re- 
duced while inhaling 10 per cent concentrations of 
oxygen. Observations were made for both stereo and 
vernier acuity. Under conditions of normal illumi- 
nation, the 12 per cent oxygen concentrations showed 
practically no effects. When the oxygen was reduced 
to 10 per cent, however, most of the subjects showed 
an increased variability in vernier acuity, with only 
slight effects on stereo acuity. Under the low illumi- 
nation of 0. 1 ft-c with 10 per cent oxygen, these effects 
were much more striking, especially the greater 
variability of vernier compared with stereo acuity. 
Under the conditions of these experiments vernier 
acuity appears to be more susceptible to the impair- 
ing effects of oxygen deprivation than stereo acuity. 

Posture 

It was believed that changes in posture might give 
rise to reduced visual performance inasmuch as this 
had already been demonstrated in the case of light 
sensitivity. It had been found that reduction in light 
sensitivity may be produced by the altered blood 
flow or altered blood pressure associated with pos- 
tural changes and may be correlated with a reduction 
in the amount of blood available for the nerve cells 


and the visual area of the brain. The basis of this 
kind of visual impairment may be the decreased 
oxidation occasioned by the lowered amount, or 
pressure, of the blood in the brain. The two Services 
employ different postures for the range finder op- 
erator-standing for the Army and sitting for the 
Navy. A study of stereoscopic range measurements 
made under these different postural positions was 
performed at the Harvard Fatigue Laboratory. (266, 
268) Experiments were carried on under a variety of 
conditions: (1) experiments with a tilt table; (2) tilt 
table experiments with the blood pressure lowered 
by administration of nitroglycerin; (3)half hour stand- 
ing periods; (4) ingestion of nitroglycerin while sit- 
ting and (5) prolonged (3-hour) periods of standing. 

A total of 84 experiments in the study of the effect 
of posture on stereo range estimates were made. The 
results indicated consistently that the variability of 
stereo range readings did not alter with (a) change 
in posture, (b) with the ingestion of nitroglycerin, 
or (c) with both changes in posture and the ingestion 
of nitroglycerin simultaneously. Only one subject 
showed an increased variability after the tilt table 
or nitroglycerin and he usually gave a poor response 
to the tilt table. Thirty-one experiments were carried 
out during periods of 3-hour prolonged standing. 
Three of the five subjects became more variable and 
showed a progressive departure from true range as 
the experiment progressed. The subject who showed 
the greatest variability and the largest departure 
from true range also gave the poorest responses to 
the tilt table. However, the observed changes in 
variability were, on the average, all less than 1 unit 
of error. Hence the general conclusion from these 
postural studies is that stereo acuity remains rela- 
tively unaffected, i.e., the changes in precision were 
less than 12 seconds of arc, regardless of the fairly 
severe physiological effects resulting from the altered 
posture imposed by these experiments. 

9 7 GENERAL CONCLUSION 

Indeed, the general conclusion to be drawn from 
the experiments outlined in this chapter of the pres- 
ent report is that stereoscopic range finding is curi- 
ously, and gratifyingly, resistant to psycho-physio- 
logical changes in the operator. This is true for fa- 
tigue, loss of sleep, drugs such as metrazol, oxygen 
deprivation, and postural changes. Hyperventilation 
produced a damaging effect of short duration and 


GENERAL CONCLUSION 


81 


benzedrine produced a salutary effect in overcoming 
fatigue. Mention has been made above of the rec- 
ommendation of the Fire Control Division of NDRC 
to the Services in regard to the use of benzedrine, fn 
regard to hyperventilation, the Fire Control Division 
recommended (22) that height and range finder per- 
sonnel should be stationed close enough to their 
instruments so that, in the case of an alert, it will 
not be necessary for them to run a distance great 


enough to produce deep and rapid breathing. Al- 
though the effects of hyperventilation, induced in 
this way, are of short duration, they might occur at 
a very critical time in the case of a sudden alert. 

ft is comforting indeed to find that the results of 
all of the other carefully performed experiments 
gave negative results, and hence the practical effects 
of these factors may be ignored in the field and dur- 
ing combat. 


RESTRICTED 
» . — — - — O 



Chapter 10 

RELATIVE POSITION OF RETICLE AND TARGET 
AND IMPORTANCE OF TRACKING 


10 1 STUDIES ON RELATION OF 
RETICLE TO TARGET 

A NUMBER OF experiments have been performed in 
several laboratories which were concerned with 
the relative position of target and reticle in stereo- 
scopic range finding. Such an experiment was carried 
on at Tufts College. (556) Records were obtained 
from six trained observers on both the Tufts Stereo- 
scopic Trainer and the Mark II Stereoscopic Training 
Instrument. The target was placed at 9 seconds of arc 
above or below the limits of the fiducial mark of 
the reticle. The results indicate that, for five of the 
six subjects, there were significant differences in the 
precision of ranging and that the results of the sixth 
subject approached statistical significance. The dif- 
ference between the means of error from readings in 
one position to means of error in the second position 
varied from 4.4 seconds to 63.3 seconds of parallactic 
angle at the observer’s eye. All of these observers had 
previously been trained, too, with the target under 
the reticle. Nevertheless the results show that for 
most subjects in this experiment, the target position 
above the reticle seemed to produce smaller constant 
errors. Three untrained subjects also took part in this 
experiment. Constant differences of similar magni- 
tude were found in the cases of two of them but not 
for the third. 

Also at Tufts College the experiments were repeated 
using the Mark II Trainer. The results indicate that 
significant differences were obtained between the 
means for seven out of ten series of observations, but 
the direction of error does not always correspond for 
the two instruments. For three observers there is the 
same tilt in the two instruments; while for three other 
observers there is the opposite tilt. The differences 
in average error for the two target positions range 
from 3 to 87 seconds for different subjects. It should 
also be noted that those subjects who showed a rela- 
tively large error on one instrument showed it on 
the other as well. 

From these results the investigators conclude that 
the target position effect discovered in the Tufts 
Trainer is also present with some subjects in the 
Navy Trainer. Those subjects whose differences be- 
tween means of target-above trials and target-below 


trials were large showed the same kind of difference 
in the Navy Trainer. These results mean that on 
both the Tufts Trainer and in the Navy Trainer 
the criterion of contact is influenced by the target 
position in the vertical meridian. 

In another experiment at the Howe Laboratory 
of Ophthalmology, six trained observers were em- 
ployed and the target was systematically shifted 
above or below the fiducial mark of the reticle in 
steps of 6 degrees, 4 degrees and 2 degrees. (306) The 
observer was allowed to bracket at will in making 
his range setting. The results indicate that each ob- 
server exhibited a steady progression in the average 
error as the target rises from 6 degrees below the 
fiducial mark to 6 degrees above; but both the mag- 
nitude and direction of the progression vary mark- 
edly from observer to observer. It will also be noted 
that the precision of the settings, as measured by 
the mean absolute deviation, is always best at the 
central position of zero separation of target and 
fiducial line in the vertical meridian. The experi- 
menters report that there was great constancy of 
ranging for each observer for observations made over 
a period of 6 months. 

In another experiment at the Howe Laboratory, 
also employing the Mark II Trainer, results were 
obtained from four subjects with the target placed 
at 0.6 degrees, 1 degree, 2 degrees, 3 degrees and 4 
degrees above the reticle. (307) Observations were 
also obtained with similar separations in free space. 
The results indicate, in general, that the trend ob- 
servable in the stereoscopic training instrument is 
essentially absent in free space. However, with the 
Trainer there is in every case a consistent error trend 
which increases as the separation between reticle and 
target is increased and, in the case of five subjects, 
becomes very large with a separation as great as 6 
degrees. 

An experiment was made at the Harvard Fatigue 
Laboratory on lateral displacement of target and 
fiducial reticle line. (264) The angle of displacement 
extended to 5 degrees on either side of the extreme 
reticle posts or 1 0 degrees on either side of the central 
fiducial diamond. Determinations were made with 
the binocular acuity apparatus developed by the 


82 


RELATION OF RETICLE TO TARGET 


83 


Fatigue Laboratory. Nine subjects each made re- 
peated determinations at 14 different separations— 
7 on each side of the fiducial marks. 

The results indicate that there are no effects of 
any considerable magnitude for any of the nine sub- 
jects when the target remains within the extreme 
lateral limits of the reticle. However, the loss of 
acuity is very great indeed when the target is placed 
outside these limits and at the extreme separation 
may become as much as 440 seconds of parallactic 
angle. Most observers show a decrease in range esti- 
mates at the extreme position on either side of the 
reticle but individual differences in the direction 
of the error were noted. At the extremes, however, 
increase in both average error and in variability of 
ranging were noted for all observers. 

No extended systematic experiments were per- 
formed on this topic at the Princeton Laboratory, 
Fort Monroe, but there were a number of observa- 
tions some of which are of particular importance, 
being made on the actual Army Stereoscopic Height 
Finder Ml.^ fn one series of observations, ranges 
were taken with the target directly beside the fiducial 
mark or with the target below the central reticle 
post. The results for three of four subjects show a 
statistically significant difference in the range when 
the target is just beside or below the central reticle 
post. In the case of all of the four highly trained 
observers there is an increase in error for the below 
position— for one subject as great as 60 seconds of arc. 
Also, for every subject, this error is still further in- 
creased under conditions of haze. It seems safe to 
conclude that one will obtain a longer range when 
the target is under than when the target is beside 
the fiducial line. 

There is also a small amount of data available 
from the Princeton Laboratory for readings taken 
on the Eastman Trainer. The target was placed 
either 1 or 2 lengths of the fiducial post directly above 
or directly below it. (372) The results indicate that 
there is a tendency to range shorter with the target 
above the reticle than with the target below and that 
this effect increases with increase in target-reticle 
separation. 

Some results from Brown University throw addi- 
tional light on the problem of target and reticle 
separation, although the experiment was devised for 
a somewhat different purpose. (164) This study was 

“These results are contained in a memorandum from the 
Princeton Laboratory dated Mav 6-8, 1942. 


concerned with a comparison of two simple types of 
reticle pattern and with an analysis of the way in 
which precision of stereoscopic setting is influenced 
by nearness of the target to a reference fiducial line. 
One group of reticles consisted of a single vertical 
line with the target set at different distances to the 
right. A second group of reticles consisted of differ- 
ent sizes of break in a single vertical line. The target 
w^as entered in the middle of such breaks. Fixed 
targets were used throughout. Hence in the first 
group, these investigators studied lateral separation 
of target and reticle and, in the second case, vertical 
separation. 

Both situations give the same results. As the fidu- 
cial line is moved farther and farther from the target, 
the precision of stereoscopic setting decreases. At 
comparable line-to-target distances, the reticles of 
the break-in-line type gave better precisions than 
the reticle of a single line placed laterally to the 
target. The average deviation for five experienced 
observers for 5 degrees separation of target and 
reticle was nearly 62 seconds of arc; in the break-in- 
line type where the widest break was slightly more 
than 3.5 degrees the average deviation was slightly 
over 30 seconds of arc. The results indicate that, in 
designing reticles, emphasis should be placed on the 
possibility of positioning the target in a break in the 
fiducial line or of presenting reticle patterns which 
give an opportunity for the target to be as near 
fiducial lines as possible. 

Much of this material has been gathered together 
in a Report to the Services issued by the Fire Control 
Division of NDRC. (8) There is no conclusive evi- 
dence regarding the cause of these effects on ranging 
of separation of reticle and target. That they are due 
to viewing through an optical system would seem 
likely from the comparison of Trainer and free space 
results of the Howe Laboratory. The suggestion has 
been made that these errors may be due to such eye 
conditions as cyclophoria, which would result in 
various tiltings of the field for different observers. 
The individual differences in degree and even direc- 
tion of the errors noted in all of the results outlined 
above from the different laboratories would tend to 
support this view. Whatever their cause, these results 
indicate the existence of errors of very considerable 
magnitude when ranging is done with target position 
in the field beyond the limits of the reticle pattern, 
either vertically or horizontally. The results are of 
special interest to the range finder operator since 


RESTR1CT0 


84 


IMPORTANCE OF TRACKING 


they point to the importance of accurate tracking of 
the target in order to maintain the position of the 
target as near the center of the reticle as possible. 

There is also a report on tracking errors from the 
Princeton Laboratory at Fort Monroe. (372) This 
contains empirical results which come from an ex- 
periment at Fort Monroe in October 1941 and also 
a theoretical discussion of the effects of such errors 
on the accuracy of stereoscopic range finding. Angu- 
lar tracking errors can cause height errors in the case 
of the Ml Height Finder in two ways: (1) an eleva- 
tion error will cause the dial reading to differ from 
true height, even though the target is correctly set 
on the reticles, and (2) irregular tracking will cause 
the target to jump about in the field of view of the 
observer, thus making observing more difficult and 
possibly less precise. This first type of error is not 
present when the Ml instrument is used as a range 
finder. Working with the M2 Trainer the experiment 
was performed with 27 student observers, with and 
without tracking errors. The performance of the class 
as a whole was significantly better, at the 5 per cent 
level, without tracking errors. How much better 
depended very much on the particular observer. The 
more accomplished observers generally showed the 
least difference under the two conditions of tracking. 
One of the quantitative results of the Fort Monroe 
test is that the standard error of elevation tracking 
is generally of the order of one mil. Hence the stand- 
ard deviation of these errors would be less than 0.6 
UOE for an altitude of 3,000 yards or greater. 

10.2 experiments on handwheel 

TRACKING 

In order to study tracking accuracies and also to 
provide an instrument to be used as a trainer for the 
tracking on the M7 Director, Tufts College devel- 
oped an instrument which was called the Tufts 
Director Tracking Trainer. (575, 577) This instru- 
ment is described and pictured in the second of these 
reports. In external appearance, general dimensions, 
position of handwheels, eyepieces, etc., it is similar 
to the actual director. The telescopes, however, in- 
stead of being pointed toward external space, are 
trained upon an internal target. This target is made 
to move along a spherical course upon a fixed center 
point and can be followed in both azimuth and eleva- 
tion by the tracking telescopes which can be rotated 
independently about the same center point. Any 


desired course can be simulated for the target by 
introduction of proper cams. Hence tracking prac- 
tice of a very realistic sort may be obtained without 
actual flying missions and, immediately after every 
run, an integrated error score may be given to the 
operator. Preliminary results indicated that the in- 
strument was valuable in the early stages of tracking 
training. However, because of the size of the instru- 
ment and its expense, it was not adopted by the Army. 

Because of these facts and other considerations, a 
detailed and systematic program of experiments for 
handwheel tracking were performed by the Foxboro 
Company. This work was undertaken because it was 
found that there was no information available on 
the limitations of performance of a human operator 
tracking with a handwheel. Furthermore, the matter 
of smooth tracking in fire control problems was in- 
creasing in importance to the point of being a major 
factor in accuracy in gun laying. Hence these studies 
have a very much wider interest and application than 
the mere centering of the target image on the reticle 
of the range finder. They are of the utmost impor- 
tance for director operators and for the following of 
a target by automatic weapons. 

10.2.1 Yjjg Human Operator 

Conditions of Experiment 

The first report of the Foxboro Company covers 
the broad phases of the problem of the human oper- 
ator and the tracking problem. (33) A description 
of the apparatus developed to study these problems 
is included. Various courses could be introduced by 
the mere substitution of cams. For most of the experi- 
ments the presentation was simple pointer matching. 
The subject, by operating a handwheel, had to cor- 
rect the movement of one pointer, actuated by the 
cam device, so that it stayed matched with a second 
stationary pointer. In some experiments, to deter- 
mine if the principles developed for pointer match- 
ing were universal for other presentations of data, the 
subjects were required to match pips on an oscillo- 
scope, a very different sort of presentation. The sub- 
jects were chosen at random from the personnel of 
the Foxboro Company, and represented a fair sam- 
pling of the plant population within the age limits 
of 18 to 35 years. Hence it was believed that their 
subjects were not different from those actually oper- 
ating such devices in the Services. The apparatus 


RESTRICTED 


THE HUMAN OPERATOR 


85 


enabled the experimenters, for every run, to obtain 
an integrated error score and also a graphic record 
of errors throughout the run. 

Learning Curve 

The first experiment was designed to study the 
learning curve of all operators and to bring each to 
his state of greatest efficiency. In every case the com- 
plexity of the learning is shown by the high error 
scores at the beginning of the experiment, by the 
irregular drops, the plateaus, and the relatively large 
number of runs necessary to reach the physiological 
limit of tracking performance. For example, for one 
group of observers the mean error score for the first 
ten runs is 192 while for the 51st to 60th runs this 
drops to 92. For another group of 10 observers the 
mean error score for the first 10 runs is 187, dropping 
to 85 for the 51st to 60th runs. In many subjects there 
is evidence that learning was not complete by the 
60th run. 

It is often said that measures of performance for 
different individuals approach each other with prac- 
tice in simple tasks and tend to diverge in more com- 
plicated ones. AVTile the absolute error scores in the 
present experiment decreased with practice, the re- 
sults show that the ratio of poorest to best operators 
does not show any such decrease. Hence the investi- 
gators conclude that there are factors making for 
more efficient performance which result in lowered 
absolute error scores and factors making for indi- 
vidual differences in tracking ability which remain 
even after considerable practice. For the last ten runs, 
for example, the mean error scores for individuals 
varied from 144 to 39. The range ratios show that 
the poorest subjects make error scores nearly 400 
per cent as great as the best. The five poorest oper- 
ators were no more accurate after 60 runs than the 
five best were after 10 runs, the former having an 
average error score of 127 at the end of this training 
as compared with 132 for the latter during the first 
10 runs. 

The graphic records of error in pointer displace- 
ment are given for good, average, and poor operators 
at the beginning, middle, and end of this training 
period. In the beginning the tracking of all oper- 
ators, and of poorer operators even after many runs, 
showed swings of large amplitude and low frequency; 
the operator turned too far to one side and then 
overshot in bringing the pointer back to zero. More 
turns of the handwheel were made than were neces- 


sary. As operators improve the swings are reduced 
in amplitude, the low frequency drift tends to drop 
out, and the line of true target position is approached 
more frequently. For example, the displacement 
curves for one of the poorest operators show she was 
practically never on or near the zero line; those of 
the better operators show frequent contacts and 
small departures from the zero line. On the whole, 
operators tend to lag behind the pointer in that their 
corrections of pointer displacements approach the 
zero line without crossing it, showing that they do 
not correct by deviating as much to the opposite side 
as they did on the original. The result is that the 
average deviation, taken with regard to sign, is not 
zero but a plus or minus quantity depending upon 
which side the lag occurred. 

The gross behavior of operators learning to track 
parallels the picture gained from the tracking plots. 
The beginner or the poor operator works harder, is 
more tense and actually expends more energy than 
the skilled tracker. The good tracker holds the knob 
of the handwheel lightly in his fingers and his arm 
and wrist are free almost to relaxation. One gets the 
impression that he is following rather than pushing 
the pointer. But good trackers are actually not taking 
it easy; they are attentively alert and thereby do the 
job with the minimum expenditure of energy. At 
the end of the training the best operators were at 
accuracy levels of tracking better than 1 mil while 
the poorest were doing not better than 3.7 mils after 
the 60 runs of practice. 

Effect of Handwheel Position 

This preliminary experiment was designed also to 
test one other factor — namely, the position of the 
handwheel as it might affect the accuracy of tracking 
a moving pointer against a stationary standard. Two 
positions of the handwheel were selected for study; 
a vertical position in which the handwheel was turn- 
ing in the plane parallel to the body of the operator 
and at right angles to the floor, and a horizontal one 
in which the plane of motion was at right angles to 
the body of the operator. From the beginning of the 
experiment the operators were divided into two 
groups. One group started with the handwheel in 
the vertical position and, after 30 runs, switched to 
the handwheel in the horizontal position. The sec- 
ond group followed the same experimental pattern 
but reversed the order for the two handwheel posi- 
tions. It was found that the differences between 


RESTRICTEl 


86 


IMPORTANCE OF TRACKING 


averages of runs of 10 between the two groups are 
not statistically reliable and hence it may be con- 
cluded that there is no permanent advantage in 
either position, even though the vertical position 
affords hand movements which more nearly corre- 
spond to desired target motion. Comparison of the 
average of the last five runs with first position and 
first five runs of second position shows that there is 
a significant increase in error score when vertical is 
changed to horizontal and a slight, but statistically 
insignificant drop when horizontal is changed to 
vertical. In both groups the downward trend of the 
learning curve was stopped, showing that any 
change in handwheel position opposed to previously 
formed habits is at least temporarily disruptive. 
However, after 10 additional runs all operators ad- 
justed to the new position and learning proceeded 
in orderly fashion. 

An analysis of individual differences in tracking 
accuracy was made of the 20 subjects who worked as 
operators in this initial experiment. Individual dif- 
ferences appeared from the beginning and were 
maintained to the end of the 60th run. The ratio of 
poorest to best operator was 3.09 for the first 10 runs 
and 3.69 for the sixth set of 10 runs. For the group 
as a whole the ratios tend to increase rather than to 
decrease, the better operators becoming relatively 
worse with practice. Also operators tend to keep their 
relative positions in accuracy of tracking. If one 
divides the 20 operators into quartiles of five each, 
one finds on comparative standing in the last group 
of 10 runs that ; (1) four of the five operators in the 
highest quartile remain in this quartile while the 
fifth moves to the second quartile; (2) four of the 
five operators in the lowest quartile remain in that 
quartile while the fifth moves up to the third quar- 
tile. These results have practical consequences. It 
appears that the best and poorest operators hardly 
change position, whereas the average (those in the 
middle quartile) change most. Furthermore the 
average operators do not, as a rule, jump to best or 
poorest positions after additional training but tend 
rather to move into continuous central quartiles. For 
the whole group, correlation between rank order 
after 10 runs and after 60 runs is 0.68; while that 
between the third and sixth group of runs is 0.85. 
This indicates that, in a sense, good trackers are born 
and not made. Hence, if it is desired to select a group 
of men who will become good trackers, some sort of 
selection procedure should be employed. For ex- 


ample, the upper 50 per cent of operators in this 
group after the 30th run would have given a range 
ratio from best to worst of 2.23; by choosing oper- 
ators from the upper quartile only, the range ratio 
drops to 1.56 instead of the 3.69 value for the entire 
group. It is possible that some of the poorer oper- 
ators could be improved with special training but 
this will not occur beyond the limits indicated by 
mere practice alone. 

The question of sex differences in these experi- 
ments has been discussed elsewhere in this report. 

These experiments were continued and the results 
are to be found in a second report from the Foxboro 
Company. (34) The conditions of presentation of 
data remained the same as those in the first experi- 
ment already described. A total of 12 men and 
women acted as operators. These were unselected 
and untrained at the beginning of the experiment. 
A third oblique position of the handwheel was in- 
troduced as well as the vertical and horizontal posi- 
tions already studied. The same pattern of learning 
curve, variability, individual differences, and sex 
differences was found for this group as for those 
previously tested. It was found that shift from verti- 
cal to oblique position of handwheel slightly inter- 
rupted the learning curve but had no greater effect 
than a subsequent shift from oblique to the hori- 
zontal position. Hence these results indicate that 
considerable latitude is possible in handwheel posi- 
tion in actual field operation. In certain pieces of 
field apparatus all three handwheel positions— verti- 
cal, horizontal and oblique will be found. 

Effect of Fatigue 

In another experiment found in this report by the 
Foxboro Company an attempt was made to discover 
possible effects of prolonged work on tracking per- 
formance. Fatigue runs were made with 12 operators 
under two sets of conditions. One group of subjects 
rested during change of error records after each 
trial, thus breaking runs with rest periods of from 
30 to 45 seconds; the other group made continuous 
runs with no break between trials whatsoever until 
a considerable number of runs had been made. Each 
of the three positions of the handwheel was used 
under both conditions of work. A total of 30 to 40 
runs were made in a single second requiring continu- 
ous operations during a period of several hours. The 
results fail to show any work decrement because of 
this continuous tracking to the extent expected from 


HANDWHEEL SPEED AND ACCURACY OF TRACKING 


87 


continuous work of this sort. In one case only is 
there an increase in error score as the work pro- 
gresses, which is the classical curve of work decre- 
ment. For five operators there is a more or less 
gradual rise in error followed by a drop so that the 
tracking at the end of the period is as good or even 
better than at the beginning. For three subjects 
there was fairly steady |>erformance with very little 
or no loss in efficiency as evidence of fatigue or work 
decrement. For the remaining three operators there 
was progressive increase in efficiency shown by drops 
in the error curve throughout the work period. 
Hence it appears that even when tracking is pro- 
longed for as long as 3 hours with little or no pause 
between runs, no breaking point or gradual loss in 
efficiency occurs, as measured by average errors. 
However, 8 of the 12 operators had greater varia- 
bility of results under fatigue conditions than was 
found for them in normal runs. Under the conditions 
of this experiment it seems that fatigue does appear 
in increased variability of tracking accuracy al- 
though average accuracy is unimpaired. 

With an eye toward selection of persons who 
would make good trackers with training, the Fox- 
boro investigators chose a type of reaction time 
experiment which might be regarded as a spot or 
work sample tracking response, involving discrimina- 
tion and choice as well as speed of reaction. A pointer 
could be made to move either to the right or left in 
random order from a position directly above a sta- 
tionary marker. The subject was given the task of 
responding to the moving pointer by pressing a lever 
similar to a telegrapher’s lever held between thumb 
and index finger. The subject was required to press 
the lever in a direction opposite to the motion of 
the pointer. The average reaction time for 28 sub- 
jects for such directional response is 406 milliseconds 
(ms). This is considerably above simple reaction 
times to light— 178 ms; to sound— 154 ms; and either 
light-or-sound— 200 ms, as obtained from the same 
apparatus. Highly significant individual differences 
were present in the reaction times of these subjects. 
A positive correlation was found between average 
tracking error for early periods of practice and re- 
action time. The correlations tend to decrease with 
more extended practice due to failure of the better 
subjects to improve beyond a certain point. It ap- 
pears as if reaction time correlates with speed of 
learning rather than finished, learned tracking per- 
formance. 


Individual Tolerance 

One extremely important factor investigated by 
the Foxboro Company and contained in this report 
is the matter of tolerance as a limiting factor on 
tracking performance. It seems that each subject sets 
up a criterion of par for himself and is perfectly 
satisfied if he remains within the limits of his self- 
determined error range. This concept of an individ- 
ual tolerance or par set up by the subject explains 
a number of facts which grow out of these experi- 
ments: (1) convergence of error scores toward a com- 
mon value of most of the subjects with larger number 
of practice periods; (2) early appearance of final 
plateau in the case of the better operators or the 
rapid learners; (3) significant differences at the end 
of learning between subjects whose scores do not 
converge; (4) absence of work decrement in accuracy 
with prolonged periods of tracking. These results 
can be readily explained on the assumption that each 
operator consciously or unconsciously adopts a stand- 
ard of performance and each is satisfied merely to 
maintain this standard. This can be accomplished 
under fatigue conditions by working harder than 
usual. Thus the aim in apparatus design should be 
to make the objective conditions such that as small 
an error will be tolerated as possible. It can be as- 
sumed that increased discrimination, motivation and 
adequacy of response may be accomplished through 
betterment of objective conditions. Any aid improv- 
ing conditions of work, making work more comfort- 
able, and bringing the task within the powers of the 
operator will increase efficiency. In the operation of 
tracking equipment, objective conditions surround- 
ing visual presentation and handwheel control may 
be improved with consequent reduction in tracking 
errors. These investigators found considerable in- 
crease in accuracy through improvement in lighting 
and background conditions, for example. 

10.2.2 Effect of Handwheel Speed 
on Accuracy 

A third report from the Foxboro Company describes 
a series of experiments aimed at determining the 
relationship between handwheel speed and accuracy 
of tracking. (35) Fourteen previously highly trained 
subjects acted as operators and handwheel speeds 
varying from one RPM to 250 RPM were imposed 


88 


IMPORTANCE OF TRACKING 


by the experimental conditions. These studies in 
direct handwheel tracking showed that the speed at 
which the handwheel is turned has a pronounced 
effect on both the amount and the character of 
tracking error. In general, and under a variety of 
conditions, high handwheel speeds were found to 
have a very favorable effect, on both accuracy and 
smoothness of tracking. For convenience in analysis, 
this series of studies was made with a course requir- 
ing constant handwheel speed for correct tracking. 
All conditions were held constant throughout the 
experiment, with only handwheel speed varying. 
The results were also confirmed by further study of 
runs on variable speed courses. These additional 
courses varied from a very easily tracked course, cor- 
responding to a simple crossing target with the cross- 
ing point remote from the operator, to a very difficult 
course, resembling a plane flying an evading course 
at close range. In all four courses were used. 

The results show that when the handwheel speed 
was reduced by 2.5 to 1, the error increased in the 
order of 50 per cent, while when the handwheel speed 
was increased, there was a corresponding decrease of 
about the same proportion. The operators appeared 
to experience no difficulty with the considerable 
accelerations which accompanied rapidly varying 
course, tracked at high handwheel speeds. It was only 
when surprise effects occurred that difficulty was 
experienced and this difficulty did not appear to be 
materially affected by the speed. The error curve 
drops steadily from 1 RPM to 50 RPM and remains 
constant at the low level through a wide range of 
between 50 RPM to the breaking point of the op- 
erator at 200 RPM or above. 

10.2.3 Inertia, Friction, and Diameter 

It was also found that friction in appreciable 
amount (2-pound friction was used), inconvenient 
position of handwheel, awkward hand grip and 
larger sizes of handwheel, all tended toward lower 
maximum speeds. Under good conditions, operators 
could track with the handwheel speed between 200 
RPM and 250 RPM. The optimal speed band within 
which minimum error occurred extended well up 
toward this maximum speed in the case of every 
operator. Within this band, variation in speed had 
little effect. For most actual courses, the variation 
of handwheel speeds exceeds the width of the opti- 


mal speed band. Hence, minimum tracking error for 
the entire course will depend upon choice of a hand- 
wheel gear ratio which will permit a safe maximum 
turning speed at maximum course speed. That is, 
the highest required turning speed must be high 
but also attainable by all operators under the par- 
ticular operating conditions. In the absence of data 
on the particular operating conditions, it appears 
safe to assume a maximum handwheel operating 
speed of 180 RPM. If a marked difference exists be- 
tween slewing speed and maximum expected track- 
ing speed, improved tracking may be expected by 
making the 180 RPM handwheel speed correspond 
to the maximum tracking speed with auxiliary pro- 
vision for slewing. 

Under the conditions studied, fatigue had little 
effect on tracking accuracy. Operators worked 15 
minutes both with and without friction. The friction 
was applied directly to the handwheel, resulting in 
the same frictional torque regardless of handwheel 
speed, thus making work done proportional to the 
speed. Under these conditions, the decline in accu- 
racy during 15 minute runs was at the low speeds 
but not at the high speeds. 

Some studies were also made with untrained sub- 
jects. The results show that the advantages of high 
handwheel speeds are as great or even greater for 
untrained personnel as for skilled operators. In re- 
gard to the character of the curves of tracking errors, 
with the low handwheel speeds there are very wide 
swings of slow frequency with the operator seldom 
on target. As handwheel speeds are increased, the 
amplitude of these swings are steadily decreased 
while their frequency is increased. At the highest 
speeds, the pattern of tracking response shows very 
small variations actually corresponding to each revo- 
lution of the handwheel but almost invariably cross- 
ing the on-target lines. The importance of these 
differences in pattern with slow and fast handwheel 
speeds can hardly be overemphasized if the data 
are to be fed into a director, predictor, or computer. 
The results show that very different apparent target 
courses could be obtained from identical courses, 
depending upon the handwheel speed with which 
the course is tracked. 

These investigators point out that, although these 
studies are of direct handwheel tracking, they appear 
to be applicable to certain conditions of aided track- 
ing as well. It would appear that with larger time 
constants, of 2 or 3 seconds or more, the feel and 


. -V ' 



INERTIA AND FRICTION 


89 


operating technique of direct and aided tracking 
are so nearly alike that the data on direct tracking 
studies are largely applicable to aided tracking with- 
out change. In particular, the lower handwheel gear 
ratios with the correspondingly higher speeds appear 
to favor greater accuracy in both cases. In aided 
tracking, the maximum speeds may not run as high 
as in direct tracking since an adequately low hand- 
wheel gear ratio with aided tracking may not require 
very fast turning even with the most difficult courses. 
Particularly, with aided tracking much more than 
with direct, the presentation of the data to the oper- 
ator may prove the limiting factor on accuracy with 
a smaller proportionate effect from the handwheel 
condition. 

A fourth report from the Foxboro Company is 
concerned with a more detailed study of inertia, 
friction and diameter of handwheel as they may affect 
direct tracking. (36) It was found that inertia, either 
in the form of a heavy handwheel or as flywheel 
effect inherent in or added to the moving system, 
reduced tracking error materially. In these studies 
a geared-up flywheel was used for low speeds, pro- 
viding a moment of inertia at the handwheel shaft 
equivalent to a 55-pound ring of 20 inches diameter. 
At a steady handwheel speed of 2 RPM this resulted 
in a reduction of error of approximately 50 per cent 
compared with a lightweight bar handle type. At 
speeds up to 15 RPM lesser flywheel effects gave 
similar results. For speeds of 20 RPM and above, 
inertia effect was provided by a heavy handwheel 
9 inches in diameter and weighing 9 pounds with 
most of the weight in the rim. Reduction of error of 
roughly 40 per cent for speeds up to 200 RPM were 
obtained with the heavy handwheel as compared 
with the light, bar type handle. The improvement 
was equally apparent for simple or variable courses. 
Even with a lot of inertia, it was only when the 
changes of direction and rate of turning were so 
abnormally great that the operator was physically 
unable to accelerate and decelerate fast enough that 
any detrimental effect occurred. It was also found 
that inertia had a very marked smoothing effect on 
the tracking performance. With inertia introduced, 
the operator was able to hold a more constant speed 
of turning when this was demanded, or to change 
turning speed more smoothly in following target 
courses. 

The investigators point out that the nature of 
inertial effect in tracking is twofold. There is the 


definite mechanical effect of inertia of opposing vari- 
ations of rotation, thereby smoothing the operating 
and reducing the tracking error. The second effect, 
less obvious but not less important, is the operator’s 
reaction to the feel of a system having considerable 
inertia. Any tendency to change the rate of rotation 
is immediately sensed as a difference of force on the 
operator’s hand and the change is avoided to a large 
extent, thereby giving a smooth turning speed. With- 
out this tactual cue to the operator, changes in speed 
occur, building up errors which are revealed in the 
presentation and are then corrected. The feel of the 
turning speed of the handwheel provided by inertia 
permits the operator to sense rate rather than to rely 
on displacement error and so helps him to do a very 
much smoother and more accurate tracking job than 
was possible simply by watching the presentation. 
This reaction to the handwheel is equally useful 
when relatively smooth constant speed is needed to 
follow the target course or where speed variations 
are required by the situation. In this latter case the 
operator can increase or decrease the force on the 
handwheel to produce the necessary change of speed 
with the reaction still useful as a guide to the amount 
of force necessary to produce the required rate of 
change of speed. A trained operator takes advantage 
of the feel of inertia to obtain improved tracking on 
irregular courses without introducing the delay nor- 
mally introduced by mechanical smoothing devices. 
This accounts for the fact that inertia is helpful even 
on a very irregular course up to the point where the 
force required to produce the necessary acceleration 
is beyond the physical limitations of the operator. 
Inertia is very easily added to any tracking system 
either by the use of a heavy handwheel, or, where 
speeds of rotation are low, by an added, geared-up 
flywheel. Inertia may be increased with beneficial 
effects up to a value where the force on the hand- 
wheel handle required to provide maximum accele- 
ration is at least 10 pounds. 

The results of these experiments show, on the 
other hand, that friction in any tracking system is 
definitely undesirable. Small amounts of friction are 
unavoidable in mechanical systems and the present 
tests indicate that the detrimental effects of these 
small amounts of friction are not serious, especially 
if there is a reasonably high inertia in the system. 
However, as the effective frictional torque at the 
handwheel increases, the adverse effect of the friction 
increases rapidly, producing large tracking errors. 


pRESTRICTEn 


90 


IMPORTANCE OF TRACKING 


This is particularly true at the lower speeds where 
the motion becomes decidedly jerky. The increase of 
friction, therefore, not only increases the tracking 
error but definitely increases the irregularity of the 
tracking. 

Just as in the case of inertia, the effect of friction is 
twofold. There is the physical effect of a force roughly 
independent of the speed of turning which makes it 
physically impossible for the operator to rotate the 
handle smoothly. There is the additional fact that 
this frictional force tends to upset the operator’s 
feel since the speed of rotation of the handwheel 
bears little or no relation to the force applied to it. 
Friction, of course, reduced the maximum speed the 
operator could turn. 

A third factor investigated in these experiments 
was concerned with the effect of the size of the hand- 
wheel on tracking. Two handwheel sizes were tried, 
4.5 and 9 inches in diameter. Both of these sizes 
are well within the reasonable range of handwheel 
sizes. As a result, both handwheels had a perfectly 
satisfactory feel. The difference between the two sizes 
did not make large differences in either accuracy 
or smoothness of tracking except when friction was 
added. However, as might be expected, the larger 
handwheel was somewhat better at the low speeds 
where the operator has difficulty in providing a 
smooth, uniform motion. The greater handwheel 
diameter provided a larger hand motion per revolu- 
tion of the handwheel and hence contributed to 
smoother tracking. Similarly, in the presence of con- 
siderable friction, the larger handwheel was advan- 
tageous, since for the same frictional torque a hand- 
wheel of double the size required only half the actual 
turning force to move it. At the other end of the 
scale, for high speeds with light load and particu- 
larly for highly variable courses, the small handwheel 
was advantageous. In this case, the motion of the 
handle of the large handwheel approached or ex- 
ceeded the physical ability of the operator, and so 
the smaller motion of the handle per revolution 
proved an advantage. 

10.2.4 Single and Double-Crank 
Handwheels 

A fifth report from the Foxboro Company deals 
specifically with the relative accuracy of handwheel 
tracking with one and both hands. (37) The results 


indicate that there was significantly superior track- 
ing performance with a two-handed crank. The 
double-crank handwheel control has certain practi- 
cal advantages in direct handwheel tracking, par- 
ticularly where the load is appreciable and where 
the steadying effect from both hands on the wheel 
is useful, as under conditions of unsteady footing. 
Since the double crank may be operated with one 
hand for short periods without very great loss in 
accuracy, the double crank provides added freedom 
of action. Also, with the two hands in opposed posi- 
tions the operator can apply more than double the 
torque he could apply with a single handwheel. This 
is useful in overcoming friction and it also makes 
possible the use of larger amounts of inertia to the 
improvement of smoothness and accuracy of track- 
ing. A double crank has the disadvantage that it 
must be mounted almost directly in front of the 
operator, low enough so that it does not interfere 
with the operator’s vision, and high enough to be 
easily reached. In contrast, a single handwheel can 
be operated satisfactorily wdthin a wide range of 
positions relative to the operator. 

In the present tests the double crank handwheel 
had an advantage of 22 per cent in tracking accuracy 
over the average of the single handwheels without 
frictional load for five operators. With frictional load 
the advantage was 25 per cent. For these particular 
conditions the single handwheel with its axis at right 
angles to the direction in which the operator was 
facing showed an advantage of 10 per cent without 
friction and 18 per cent with 7 pounds friction, over 
the single handwheel with its axis parallel to the 
directions in which the operator faced. Again the 
pointer matching was the form of presentation and 
unidirectional courses were employed. 

Target Speed 

Still another Foxboro study analyzes the effects of 
target speeds and rates of turning on accuracy of 
direct handwheel tracking. (219) It was found on 
tracking a moving target with a hand crank, over 
a wide range of conditions, that tracking accuracy 
can be improved both by reducing gear ratio and by 
increasing the crank radius to increase the hand 
motion corresponding to a given movement of the 
presentation. Accuracy of tracking decreases with 
increasing speed of target, but not in direct propor- 


RESTRICTFD 


Ri7\ 


EXPERIMENTAL TRACKING TRAINERS 


91 


tion. For a given crank radius the gear ratio which 
gives best results at the maximum or limiting speed 
also gives better results at all lower target speeds 
than any other ratio which could be used at the 
limiting target speed. From these results the follow- 
ing conclusions are drawn: 

1. Accuracy of tracking is greater with hand and 
arm motions near the operator’s limit of turning, 
whether induced by lower gear ratios or larger sized 
handwheels. 

2. Gearing down to increase accuracy is about 1.5 
times as effective as increasing size of handwheel. 

3. The maximal target speed to be followed limits 
the gear ratio and radius of handwheel since turning 
speed must be kept below the breaking point. 

4. In the present experiments, optimal size of hand- 
wheel was found to be at least 8-inch radius, which 
was the largest employed. Since the angular breaking 
speed is reached sooner with this size handwheel than 
in the case of smaller, cranks of not more than 6-inch 
radius and not less than 2-inch radius are recom- 
mended for maximum range of good turning speeds. 

10.2.6 Magnification of Data Presentation 

Another Foxboro Company report deals with the 
improvement in direct, aided, and velocity tracking 
through magnification of data presentation. (220) 
In this study improved tracking accuracy from higher 
magnification of presentation is of primary interest 
in both direct and aided tracking by means of hand- 
wheel-operated control. The results indicate that 
tracking accuracy is definitely increased with higher 
magnifications in the case of direct tracking of a 
constant, unidirectional course, and to a greater 
extent in difficult courses requiring high velocities 
and accelerations as well as reversals in turning. 
Through the introduction of simulated noise, it was 
found that when magnification increases jitter and 
unsteadiness to the point where the presentation is 
bad, lower magnification is preferable in keeping 
with the principle that elimination of eyestrain im- 
proves overall tracking. Magnification of presenta- 
tion data was found to be of greatest benefit with 
aided and velocity tracking probably because oper; 
ators are able, by means of aiding, to exercise better 
manipulative control in accordance with what they 
see. Finally, it was found that individuals approach- 
ing tracking for the first time do better with higher 


magnifications immediately and that change in 
magnification factor during training expedites learn- 
ing to track. For the aided tracking situation with 
aided constants varying from 0.02 to 4.5 seconds the 
errors curves are parallel and the errors are reduced 
to about one-third for 4x over unit magnification. 

10.3 EXPERIMENTAL TRACKING 
TRAINERS 

A tracking trainer was devised at Tufts College 
as an experimental instrument for studying the track- 
ing problems which occur in the operation of the 
Army Directors M7 and M4. This instrument is 
completely described in a report. (87) It is similar 
in external appearance and general dimensions to 
the Director M7, and duplicates those character- 
istics which are essential to the tracking operations, 
such as, positions of handwheels and eyepieces, hand- 
wheel ratios and torque, rotation of instrument case 
and telescopes, and the like. The instrument is pro- 
vided with its own internal target which is an illu- 
minated photographic slide. The position of the 
target is controlled by cams which can be made to 
duplicate any desired aerial course. Hence, true 
target position is always known. Gourses can be 
tracked by both the azimuth and elevation trackers 
simultaneously or the instrument can be used for 
testing either tracker alone. An ink-recorder in the 
trainer provides a continuous record of the errors 
made by each of the two operators. In addition, a 
clock score is obtained which represents directly the 
amount of time “off-target”. These immediately 
available measures of performance are useful both 
from an instrumental and a research point of view. 

Another report outlines the results of the use of 
the Tufts Tracking Trainer as a selection and train- 
ing device for trackers on the M7 Director. (88) 
Studies of both reliability and validity were ob- 
tained. Two groups of 16 untrained men were 
matched for initial tracking ability on the basis of 
a pretest on the Tufts Trainer. Each group was given 
a total of three hours of distributed tracking prac- 
tice, one group on the trainer and the other on the 
M7 Director. Final tests for both groups were then 
conducted on the director during which photo- 
graphic records were taken of tracking performance. 
The average error scores obtained from the pretest 
and final test courses tracked on the trainer were 
found to be very reliable. Scores from a pretest of 


92 


IMPORTANCE OF TRACKING 


6 minutes duration were found to predict success 
in tracking on the Director M7, with a degree of 
accuracy high in comparison to the general run of 
selection tests in use in the Services and in industry. 
It was also found that 3 hours practice in tracking 
on the trainer resulted in at least as accurate track- 
ing on the director as the same amount of tracking 
on the director itself. Although the average decrease 
in error resulting from three hours training showed 
improvement from 3.95 mils to 1.51 mils, neverthe- 
less the learning curve indicates that learning was 
still not complete at the end of this period of training. 

10.3.1 Antiaircraft Tracking 

Some special studies of antiaircraft tracking were 
performed at Iowa State College in an attempt to 
consider the human operator in his servo capacity, 
with special emphasis on the design of the associated 
apparatus by which the servo action is obtained. 
(315) A job miniature was produced both with direct 
and aided tracking. A handwheel, single and double 
finger knobs were introduced as tracking controls. 
The learning curves indicate that the tracking error 
for handwheel is largest followed by single knob 
and lowest with double knob controls. Hence it 
would seem that the use of two hands, such as one 
employs in the double knob control, gives better and 
more continuous control. An aiding ratio, under 
these conditions of experiment, with an aiding value 
of 0.071 seconds proved to give a smaller tracking 
error than one of 0.125 seconds. Other factors, such 
as friction, were also studied. Some psychological 
factors of the operators were also investigated. 

From these results the investigators conclude that 
subjects should be selected as trackers after consid- 
erable practice on the actual field apparatus or a 
good job miniature. It is believed that a selection 
program outlined should yield 10 per cent of the 
original number of candidates who could track with 
errors well under 40 per cent of the original group 
average. Double fingerknobs of 2-inch diameter are 
distinctly better as manual controls than the hand- 
wheels now in use, giving only about 65 per cent as 
much error. This superiority persists with a loading 
resistance of 1500 gm— cm. If an aided rate follower is 
used, the time constant seems quite independent of 
the angular velocity at crossover and of the type of 
manual control used. The most advantageous value 


seems to be R = 0.17-0.20 seconds, which is near the 
Kerrison value. 

In a supplementary report for Iowa State College 
(3 1 6) special attention is given to problems of follow- 
ing a target with acceleration and of magnification. 
It was found that the use of a component of accelera- 
tion considerably reduces the tracking error. The 
most advantageous direct component is fairly inde- 
pendent of the accelerational term. It was found, 
also, that an increase of magnification from 30x to 
75x was not helpful while a reduction to 12x was 
definitely a hindrance. 

A second supplementary report from the Iowa 
State College contains results on two additional 
tracking problems. (317) The first of these indicated 
that a 4-, 6-, or 1 0-week layoff did not affect tracking 
accuracy, provided the subjects had been previously 
very highly trained. No differences were discovered 
when trackers used binocular or monocular vision. 

10.3.2 Comparison of Sights for Slewing 
and Tracking 

At the request of the Naval Bureau of Ordnance, 
the Foxboro Company made a comparative study of 
the slewing times and of tracking errors with several 
kinds of sights used with small caliber antiaircraft 
weapons. The first report deals with the comparative 
slewing times with optical ring, telescopic, and ring- 
post sights. (222) The results indicate that the opti- 
cal ring sight has a marked and highly statistically 
significant advantage in the time required to get on 
target. The length of the slewing paths varied from 
5 degrees to 45 degrees and also varied greatly in 
direction. Six inexperienced operators were used in 
the tests. It was found that differences in slewing 
time as large as 0.5 second appear with the use of 
different types of sight. Quickest slewing, on the 
average, was found with the optical ring sight 1.44 
and 1.26 seconds for azimuth and elevation respec- 
tively, and slowest slewing for the telescopic sight— 
1.87 and 1.75 seconds. The ring and post sight falls 
between these two on speed of slewing after initial 
practice, during which ring and post is about as slow 
as the telescopic sight. Slewing distance, obviously 
was found to be important in determining slewing 
times, there being progressive increase in time with 
increase in slewing distance. However, the time in- 
crement, within the limits used, is not as great as the 
distance increment since the increase in time is only 


c 


RESTRICTED 


3 


TRACKING EXPERIMENTS 


93 


about one-third whereas the increase in distance is 
ninefold. Since the reaction time merely to start the 
slew at a given signal is only about one-tenth the 
total slewing time, most of the time spent in slewing 
must be taken up wdth finding and getting on target. 

In a second report from the Foxboro Company, 
tw^o additional sights, a smaller optical ring and an 
illuminated ring sight, were added to the comparison 
of slewing times. (224) The present tests in general 
confirm the previous findings: slewing times are 
shortest with optical ring sights, whether large or 
small radius, longest with ring-post and telescopic 
sights, and intermediate with the illuminated ring. 
The two sizes of optical ring gave practically identi- 
cal results so far as slewing time is concerned. The 
difference between the best and poorest sights for 
slewing amounts to as much as one-half second which 
means from 33 to 50 per cent of slewing time, depend- 
ing upon the time taken as base. Comparisons of the 
sights with light and heavy mountings showed no 
differences in slewing time due to inertial effects. 

The Foxboro Company reports data from an ex- 
periment to test the accuracy of tracking with illu- 
minated, telescopic, ring-post, and optical ring sights. 
(223) Under the conditions of these tests, the 10 mil 
optical ring, telescopic, and illuminated sights were 
equally accurate, but superior to the 50-mil optical 
ring and ring-post sights. The 50 mil optical ring is, 
however, superior to the ring-post sight. A bright sky 
background and moderately difficult tracking course 
w'ere characteristics throughout these experiments. 
The average tracking errors in mils were between 
2.8 to 3.1 seconds for the three best sights, 3.8 for the 
50 mil optical sight and 7.6 for the ring post sight. 


A second Foxboro Company report summarizes 
additional tracking experiments using the same five 
sights. (225) The previous report dealt with experi- 
enced observers working in bright daylight. The 
present report is concerned with the accuracy of the 
same sights when tracking under two further condi- 
tions: (1) with experienced operators in simulated 
twilight as well as daylight, and (2) with inexperi- 
enced operators in daylight only. The results of this 
experiment confirm the previous finding that for 
trained operators the 10-mil optical ring sight, the 
telescopic and illuminated sights yield equally accu- 
rate tracking. Both studies and all operators agree 
in showing the 50-mil optical ring to be less accurate 
than the 10-mil optical ring but superior to the ring- 
post sight, while the latter in turn is much the poor- 
est of all. The ring-post sight proved to be even worse 
with the untrained than the trained operators, giving 
an average tracking error of 13.8 mils. Under the 
simulated twilight, with low illumination of less 
than 1 foot candle, the tracking with the 10-mil 
optical ring was adversely affected, but this was 
probably because the particular sight used in these 
tests lacked a clear central area. Only the telescopic 
sight proved to be significantly better than the other 
sights under the low illumination. The ring-post was 
significantly worse than all the other sights under 
the low illumination. The results with untrained 
operators indicate the need for additional practice 
to attain comparable accuracy when using sights 
which lack a center dot. The optical ring sights, 
however, do not require much extra practice to com- 
pensate for lack of center dot and thus that lack is 
not so serious in their case. 


I RESTRII 


Chapter 11 


EFFECTS OF HAZE AND OF ATMOSPHERIC 
SCATTERING 


111 INTRODUCTION 

I T IS a generally recognized fact that objects further 
away from an observer seem less clear and distinct 
than objects which are closer. This effect may be 
seen monocularly and, indeed, is one of the cues used 
in judging distance. The effect is due to the fact that 
there is more intervening air between the observer 
and the distant target. This effect is well known to 
psychologists and has been called aerial perspective. 
It is a technique universally employed by artists to 
accentuate apparent depth in their paintings. A 
corollary of this situation is that, if two objects are 
the same actual distance from the observer but one is 
clear and the other indistinct, the clear object will 
seem to be apparently closer to the observer. 

Such a situation is found in the range finder when 
the weather conditions are such that haze intervenes 
between the observer and the target. In this case the 
reticle will appear clear while the target appears hazy 
and indistinct. It has been found that this monocular 
cue may have an effect upon the binocular stereo- 
scopic judgment and hence give a false range. This 
fact is recognized in the Navy by the determination 
of what has been called Curve B. It was known that 
the correction necessary to obtain true range might 
vary, not only with the distance of the target, but also 
as a function of the particular range finder and of 
the particular operator. Methods are outlined for 
determining the influence of each instrument and a 
corrected adjuster setting is introduced to overcome 
this error. 

Some field studies were carried on at the Princeton 
Laboratory at Fort Monroe to study this effect. The 
effect of weather factors on the RCS setting used is 
reported. (373) 

Another Princeton Laboratory study was made to 
determine, for the stereoscopic height finder when 
employed as a range finder, (1) the extent of errors 
in range determinations on fixed targets; (2) the 
amount of errors arising from certain selected defi- 
nite causes for which calibration was possible; (3) 
the amount of residual error which cannot be cor- 


rected by calibration; and (4) the causes responsible 
for this residual error. (26) In a typical experiment 
ten observers made a total of 110 range determina- 
tions (each the mean of ten single readings) on five 
targets, using three different instruments. It was 
found that the standard deviation of these determi- 
nations (i.e., the standard error of a single determi- 
nation) was 15 UOE. In this experiment, systematic 
differences as great as 17 UOE were found between 
instruments; as great as 16.5 UOE between individ- 
ual observers, and as great as 9 UOE between targets. 
The elimination of these systematic differences left 
a residual variation, expressed as the standard error 
of a single range determination, of about 6.25 UOE. 
These results are typical of a much larger body of 
data, taken at Fort Monroe, on 20 highly trained 
range finder observers with a total of about 30,000 
individual readings. The causes of these errors show 
that errors in RCS account for at least 1 to 2 UOE 
and that this is partly due to the operator’s method of 
making the setting. Errors of this magnitude are re- 
ported in other experiments. It appears that certain 
observers gave reasonably regular curves but that 
there are marked differences in observers, both in the 
slopes of the curves and in the scatter of the individ- 
ual readings. It was also evident that, for some 
observers, the slopes of the correction curves differ 
with different instruments. Another report gives a 
new formula for converting range errors into UOE 
which is a better approximation, particularly at the 
longer ranges, than the one commonly used. (426) 
One extremely interesting and important research 
by the Fort Monroe Princeton Laboratory had to do 
with the changes in Curve B with time. (427) Data 
were calculated for three test observers over a long 
period of time and after this large amount of prac- 
tice, with one exception, there is no longer any indi- 
cation of anything but a flat correction independent 
of range. These data are too small to be a basis for 
generalization; they are, however, highly suggestive. 
Because of the very large amount of practice of these 
observers on fixed targets, it seemed reasonable to 
suppose that their accuracy had improved. Hence it 


EFFECT OF HAZE 


95 


seems reasonable that, insofar as Curve B is a result 
of conflict between stereoscopic cues and other cues, 
this continued observation should result in improv- 
ing the operator’s ability to make judgments solely 
on a stereoscopic basis and to ignore the monocular 
cues of aerial perspective. In other words. Curve B 
may be trained out of some observers by continual 
practice. 

112 STUDIES ON EFFECT OF HAZE 

In order to study the effects of haze under more 
controlled conditions, a method of producing simu- 
lated artificial haze in controlled amounts was devel- 
oped at the Princeton Laboratory at Fort Monroe. 
(428) A small amount of data taken in this manner 
were consistent with the belief that Curve B can be 
explained in terms of aerial perspective. At least it 
was determined that range settings became consist- 
ently longer with the introduction of haze. The 
description of a model of an instrument for produc- 
ing simulated artificial haze is reported. 

Loss of Contrast 

A final experiment on this topic at the Fort Mon- 
roe Princeton Laboratory has to do with diffuseness 
and lack of contrast in the atmosphere and their 
possible effects on range reading. (429) This is a 
theoretical discussion of the effects of atmospheric 
scattering as against poor seeing, which is believed 
to result from more or less random fluctuations in 
the refractive index of portions of the atmosphere 
due to the passage of waves of pressure or tempera- 
ture. The conclusion drawn from this study is that 
the effect of atmospheric haze was found to be con- 
centrated on the production of loss of contrast rather 
than of diffuseness. The opposite is true of the effect 
of poor seeing. It is pointed out that the meteorologi- 
cal correlation between these effects might easily lead 
to their confusion. 

One other field experiment, which has some inter- 
est for coincidence types of instruments, was reported 
by the Howe Laboratory of Ophthalmology. (313) 
In order to obtain field data on real haze, a larger 
contour break apparatus was constructed with a 
target face 5 feet square. Three degrees of contrast 
approximately 85, 50, and 25 per cent were available. 


Observations were made through 12x binoculars and 
the apparatus was set up at several beaches and 
other observation points. A typical result shows that 
at a distance of 5 miles a break subtending an angle 
of 12.7 seconds of arc was actually observed whereas 
it was calculated that a break of 10.8 seconds of arc 
would have been expected had the target been 
viewed through a vacuum at this distance. In this 
instance, the effect of the atmosphere on contour 
break appears to be very slight. Loss of contrast was 
quite noticeable and at distances beyond 5 miles the 
target as a whole could not be distinguished from 
the shore background. There were also results which 
indicated that stereoscopic accuracy on large high 
contrast targets was also relatively unaffected by dis- 
tance as long as the targets could be seen. Observa- 
tions taken on quite hazy days showed more marked 
deterioration of performance. Observations appeared 
to be more variable and the deviation from linearity 
less marked away from the sea, possibly because of 
smoke. 

A highly controlled and carefully conceived series 
of experiments to determine the effects of haze and 
of other factors which might produce constant errors 
in range were performed at Ohio State University. 
Three aspects of haze were studied, loss of contrast 
of target with background, blurring of target out- 
line, and the chromatic difference of target and 
reticle backgrounds. 

In regard to the effects of loss of contrast between 
target and background, the measurements indicate 
that there may be a Curve B effect (increase in appar- 
ent distance of target) in some subjects when the 
contrast is reduced to only 50 per cent. (336) Almost 
all subjects show the constant error when the con- 
trast is reduced to near threshold values, as would be 
the case when ranging on objects near the visual 
range. In general, the constant error increases as an 
exponential function of decreased contrast. All sub- 
jects indicate an increase in the apparent distance of 
the target when conditions of seeing are made more 
adverse, by reducing the brightness level in addition 
to a reduction in contrast. The loss of contrast is 
effective for a number of relations of target, back- 
ground and reticle brightness, simulating various 
conditions of ranging. It was found that the effect 
of loss of contrast can be reduced to some extent by 
training, especially in those individuals who show 
the effect at relatively high levels of contrast. How- 


/RKSl KlCTTr 


96 


HAZE AND ATMOSPHERIC SCATTERING 


ever, the nature of the relation between contrast and 
range error remains much the same. 

Blurredness 

In studying the effects of blurredness, it was found 
in the experimental situation that it was possible to 
produce a Curve B effect by blurring the edge of the 
target. However, calculation of the effect of fog on 
the blurredness of a real target indicated that the 
atmospheric scattering due to fog is usually a factor 
in reducing effective contrast rather than actually 
blurring the brightness contour which defines the 
target. Therefore, it is concluded, blurredness as such 
is probably not a significant aspect of the constant 
errors introduced by fog and haze. Of much greater 
importance is the loss of contrast between target and 
reticle. In another report, there is developed in detail 
a theory of the effect of atmospheric scattering and 
absorption of light by the atmosphere and the effects 
upon the appearance of a dark object against a sky 
background. (335) This hypothesis forms the theo- 
retical basis for the experiments reported above. 

As a result of these experiments, the Fire Control 
Division of NDRC made the following recommen- 
dations. (14) Variability in the personal constant 
error shown with loss of contrast makes it difficult 
to correct for this factor. Nevertheless, if it is found 
that the height and range finder must be operated 
under conditions of reduced contrast, it would be 
worth while to determine the extent of the effect for 
each operator and to measure the contrast between 
target and sky (or other background) directly at the 
time of making the range, and thereby make possible 
a correction for this factor to determine true range. 
This could be accomplished perhaps by mounting 
a light meter or other device for measuring contrast 
directly on the range finder. 

In a further laboratory experiment at the Ohio 
State University it was demonstrated that the extent 
to which blurredness impairs the sensitivity of a dif- 
ference in distance is dependent upon the size and 
shape of the target. (327) In the case of a large object, 
there is no loss in contrast between the center of the 
blurred image of the target and the background, 
whereas, in the case of narrow lines, the contrast 
decreases in inverse proportion to the addition of 
lens power to the distance correction, in the experi- 
mental instrument. It is pointed out that the width 
of the reticle marks in the Ml Height Finder are in 
the class of objects which would be subject to a loss 


in contrast as a result of increased blurredness. This 
indicates that great care must be exercised in the 
focusing of the eyepieces of the instrument to obtain 
sharp and clear reticle patterns during ranging. On 
the other hand, a bomber at 10,000 feet is in the class 
of objects which would not be affected by small 
degrees of blurring introduced by errors in the 
optical system, reduced apertures, and improperly 
focused eyepieces, so far as contrast with the back- 
ground is concerned. Smaller planes at greater dis- 
tances would certainly fall at the borderline between 
these two classes of objects. 

Following the suggestion of the NDRC report, 
Ohio State University devised a combination track- 
ing telescope and contrast meter for a stereoscopic 
range finder. (333) A vectographic reticle was devised 
for rotating a vectographic film in front of a constant 
polarizing disk in connection with the crossed lines 
of a tracking telescope. After trial of several modi- 
fications, this was abandoned and a 12-spot reticle 
was developed instead. In the reticle of the tracking 
telescope to be used for azimuth tracking, 12 circu- 
lar dots are placed, corresponding to the hour posi- 
tions on a clock face. These are produced photo- 
graphically and transmit amounts ranging from 
about 94 to 4 per cent of the light transmitted by the 
clear portion of the reticle. Hence the contrast of 
these spots varies from 6 to 96 per cent. The azimuth 
tracker merely has the task of determining which 
spot most closely resembles the target. Two reticles 
were prepared. Detailed descriptions of their manu- 
facture, preparation, and mounting and use are 
given in the report. 

The Ohio State Contrast Meter was given field 
trials at the Naval Training Schools at Fort Lauder- 
dale, Florida, and the results are reported by the 
Applied Psychology Panel. (93) This report con- 
cerns the problem of the change of range error oc- 
curring as a result of change in the contrast made by 
an aerial target with its background when seen 
through a stereoscopic range finder. In order to test 
the hypothesis that range error changes in a positive 
direction whenever contrast of target with back- 
ground becomes poor, ranges on aerial targets were 
taken by 42 observers in Class 8 at this station. Simul- 
taneously with the ranges, contrast readings were 
made by means of the Ohio State Contrast Meter, 
and reference ranges were determined by means of 
Mark 10 radar. From these reference ranges, range 
error was computed. 

Two analyses of the data were conducted, one for 


c 


RESTRICTED 


3 


CHROMATIC DISPERSION OF THE HUMAN EYE 


97 


the entire group of 42 observers, the other for a 
group of 1 1 men who had recorded 1 0 or more ranges 
at both high and low extremes of contrast. In neither 
case did a comparison of the range errors at high 
and low contrast show a statistically significant dif- 
ference. It is true that the average curve of range 
error runs from a slightly minus to a somewhat plus 
value (—0.1 to -1-2.5 UOE for the best 11 observers). 
Still, the variability of individual or even of average 
observations of the different observers makes this 
trend statistically unreliable. Because of the possi- 
bility that a significant effect of contrast on range 
errors had been masked by the effect of a third vari- 
able, namely range, a further analysis was made. This 
indicated that such a masking effect was not present 
for the group of 42 observers. If the range variable 
had any effect, it was to increase rather than to de- 
crease any tendency toward positive range error due 
to low contrast. 

The actual contrast which the aerial target made 
with its background varied radically, in a large pro- 
portion of the cases, over a short run of 140 seconds. 
This would make any possible correction for con- 
trast very difficult even if the necessity for such cor- 
rection could be shown. Under conditions of contrast 
variability less subject to quick changes than those 
of this experiment, some correction might be ap- 
plied. The contrast conditions under which the data 
for this report were collected show no significant 
effect on range error, and allowed no possible cor- 
rection to be calculated. 

11.2.3 Chromatic Dispersion of the 
Human Eye 

Ohio State University also performed a very care- 
fully controlled laboratory experiment on the chro- 
matic dispersion of the human eye and its possible 
influence on stereoscopic range finding. (334) Meas- 
urements were made of the ocular chromatic dis- 
persion for colored targets with a wave length dif- 
ference of 93 millimicrons. The values ranged, for 
the 13 subjects, from 102.4 to —19.7 seconds of arc. 
Measurements on two individuals indicate that the 
chromatic dispersion may change by as much as 28 
seconds when the field brightness is changed to pro- 
duce a pupil constriction of the order of 1 to 2 mm. 
The authors believe that this change is probably re- 
lated to eccentric constriction of the pupil. In a situa- 
tion designed to simulate the chromatic aspects of 
haze, constant errors were found, apparently depend- 


ent on the color difference of 45 millimicrons be- 
tween target and reticle backgrounds. For the same 
13 subjects, the values ranged from 54.8 to —25.8 
seconds and correlated fairly well with the measure- 
ments of binocular dispersion indicated above. Final- 
ly, calculation of the difference in color of the target 
and reticle backgrounds under haze conditions indi- 
cate that a color difference as great as that used in the 
experiment simulating haze effects could be present 
and that, therefore, the ocular chromatic dispersion 
might be an important factor in producing constant 
errors in range finding. 

As a result of this experiment, the Fire Control 
Division of NDRC made the following recommenda- 
tion. (13) It was recommended that measurements 
be made, using a simple portable apparatus, of the 
ocular chromatic dispersion of a group of range 
finder operators. These data could be compared to 
the operators’ constant errors in haze and fog to 
determine whether the dispersion factor is influential 
in producing these errors. If such a relationship is 
established, it would be a simple matter to screen out 
operators, prior to training, who show a large 
amount of chromatic dispersion. There is no evi- 
dence that this recommendation was ever adopted 
by either Service. 

A further study of the relation of chromatic aber- 
ration and dispersion of the eye to the blurredness of 
an object seen through the Ml Height Finder was 
performed at Ohio State University. (326) This is a 
theoretical discussion of axial chromatic aberration 
and chromatic dispersion of the eye as might be 
found in the use of this instrument. Several available 
methods for overcoming the influence of this factor 
are outlined. It is pointed out that chromatic dis- 
persion may be controlled by (1) decentering the 
eye with respect to the exit pupil of the instrument; 

(2) the use of a filter that is relatively monochromatic; 

(3) changing the size of the exit pupil; and (4) under- 
correction of the eyepieces. 

An experiment was performed at Camp Davis to 
study Service personnel in regard to chromastereop- 
sis and to test several instruments to determine the 
amount of individual influence by several devices 
which were developed. (331) Direct measurements 
were made with a rotary dispersing prism which is 
described in the text. An indirect measurement of 
chromastereopsis was made by comparing the inter- 
pupillary distance with the separation of two slits 
required to compensate chromastereopsis. Pooling 
the results of several groups of 30 subjects each (all 


RFSTRh 


98 


HAZE AND ATMOSPHERIC SCATTERING 


students at the Stereoscopic Height Finder School) 
the average for chromastereopsis is 31.7 seconds of 
arc. Measurements found with the split gauge are 
on the average about 0.5 mm smaller than the aver- 
age interpupillary distance as measured with the 
Shuron IPD gauge used in connection with a mirror. 
This corresponds to about 116 seconds of chro- 
mastereopsis. Inasmuch as the majority of the op- 
erators show a definite tendency to a positive type 
of chromastereopsis, most of them would have to be 
screened out if stringent tolerances were placed on 
chromastereopsis. It seems more desirable, therefore, 
to provide compensation for chromastereopsis by 
making use of various accessories for the range finder 
than to try to screen out persons who show an appre- 
ciable amount of chromastereopsis. However, chro- 
mastereopsis varies too much from individual to in- 
dividual to assume a fixed allowance for all observers. 
The several methods for compensating for chro- 
mastereopsis are discussed, as well as the chromatic 
properties of the several parts of the standard range 
finder. 

Intermittent Visibility of Low 
Contrast Targets 

Ohio State University has also performed a very 
careful experiment to determine the effects of the 
intermittent visibility of low-contrast targets on 
stereoscopic range measurements. (325) With targets 
of low contrast, they have demonstrated that there 
are periods during which only one eye sees the target, 
because of intermittent activity at the peripheral 
levels of the visual apparatus. During such periods 
of monocular visibility, it was found that there is a 
predominant tendency to see the target as lying be- 
hind the plane of the reticle. Unfortunately the ob- 
server is not aware of such periods of monocular 
vision when using a binocular instrument. The in- 
troduction of intermittent exposures to alleviate this 
situation proved only moderately successful. How- 
ever, the situation occurs only with targets of very 
low contrast. This effect is produced because the 
activity of each eye is independent of the other. The 
most likely explanation of the fact that the target 
is seen more remote than the reticle during the times 
of monocular viewing is that the reduced contrast 
of the target serves as a cue for seeing it at an in- 
creased distance, and this cue is allowed to have 
full sway when the stereoscopic cue is removed. 


An investigation was made of the possibility of 
extending the range for making usable range meas- 
urements by resorting to intermittent exposures. 
The investigators designed a clever apparatus for 
this purpose which might have been incorporated in 
a range finder. The principle behind this scheme was 
to take advantage of the capacity of a new onset of 
stimulation to initiate a visible impression of a faint 
target which tends to disappear periodically when 
the exposure is continuous. This scheme also had the 
advantage of synchronizing the effects for the two 
eyes. Although intermittent exposure has the effect 
of such synchronization, a level is soon reached at 
which, with intermittent exposures, the target is no 
longer visible with every flash. When this level is 
reached the same situation obtains with intermittent 
exposure as with continuous exposure in that, with 
some of the flashes, the target is seen with only one 
eye. An exposure of at least 1-second duration is 
indicated as the minimum that can be tolerated 
for making stereoscopic measurements. However, 
increasing the length of the exposure would intro- 
duce other complicating factors, because the natural 
bracketing rate of a range operator is about 1 to 2 
seconds per cycle. In this connection, attention is 
called to the fact that the precision of settings at 
higher levels of contrast is impaired by the use of 
intermittent exposure, which is effective only at low 
levels of contrast. 

Finally, the group at Ohio State University in- 
vestigated the relation of stereoscopic acuity to ac- 
commodation and vergence. (328) These investi- 
gators demonstrated that the maximum stereoscopic 
acuity occurs in the region of zero vergence and small 
accommodation changes. Explanation of this state 
of affairs is made in terms of the blurredness of the 
retinal image. Since the axes of the range finder are 
adjusted for substantially zero convergence, it fol- 
lows that range finder operators will not reach their 
maximum level of stereoscopic acuity if they have a 
substantial amount of heterophoria, so that they are 
required to exert substantial vergence effort. 

The last several papers from Ohio State University 
were abstracted in a report to the Services issued by 
the Fire Control Division of NDRC. (32) No recom- 
mendations were made to the Services on the basis 
of these reports. Although some of these studies 
demonstrated sources of error in the use of the pres- 
ent stereoscopic instruments, it was believed that the 
magnitude of such demonstrated errors was rela- 
tively small compared with other known factors. 


RESTRICTED 


Chapter 12 

MISCELLANEOUS FACTORS OF OPERATION 


12 1 CONTINUOUS CONTACT VS 
BRACKETING TECHNIQUES 

A T TUFTS COLLEGE a laboratory study was made to 
Jtx. determine the cause of the phenomenon of con- 
stant error, the tendency to set contact with the target 
either in front of or behind the fiducial mark. (558) 
Four bracketing methods were employed: (1) Sub- 
ject was instructed to move target so that he obtained 
a break on the opposite side from that on which he 
started, then to break on the same side, then to set the 
midpoint of this interval. Only two changes in direc- 
tion of movement of the target knob were allowed. 
(2) The subject was instructed to move the target 
so that he obtained a break on the opposite side and 
then to make contact by reversing the direction of 
movement of the target knob. Only one change in 
direction of movement of the target knob was al- 
lowed. (3) The subject was instructed to make contact 
by moving the target knob in one direction only. No 
changes in direction were allowed. (4) The subject 
was allowed to use any bracketing technique he cared 
to. Unlimited time to make contact was allowed. Six 
highly trained subjects were used on the Tufts Stereo- 
scopic Trainer. The results do not indicate any statis- 
tically significant differences in constant error or in 
variability score among any of the four methods; 
unlimited bracketing is no better than direct contact 
for the fixed targets which were employed. The in- 
vestigators conclude that height and range finder op- 
erators, after training, should be encouraged to set 
contact directly without bracketing. This procedure 
would cut down on dead time without, apparently, 
significant loss of precision in setting contact for fixed 
targets. 

A laboratory study was conducted at the Howe 
Laboratory to determine the effectiveness of make- 
and-break and continuous tracking in range at vari- 
ous rates of change of angular disparateness. (308) 
A complicated apparatus is described in the appen- 
dix of this report. With this apparatus it was possible 
to simulate in five short ranges from 25,000 to 5,000 
feet and hence simulate the path of an incoming tar- 
get diving at apparently 300 knots. The ranging 
mechanism was built so that it essentially reproduces 
the regeneration effect of a Mark 42 Range Finder 


employed in conjunction with a Mark 1 Computer 
and rate-of-change-of-range receiver. Without the re- 
generative feature, results were made for five trained 
subjects making contact; 1 front to back, back to 
front, and continuous tracking. Subsequently various 
amounts of regeneration were introduced into the 
experimental situation. 

The results indicate that, as the amount of regen- 
eration increases, thereby reducing the rate of change 
of disparateness, the advantage of front-to-back con- 
tact decreases until finally continuous tracking may 
become slightly superior. The investigators conclude 
that if make-and-break contact is employed on an 
incoming course, greater accuracy will result if front- 
to-back rather than back-to-front contact is employed, 
particularly when the rate of change of disparateness 
is high, before regeneration is made available. After 
a rate has been established and regeneration is ade- 
quate, alternate front-to-back and back-to-front con- 
tacts will tend to reduce constant range errors. How- 
ever, this is not to be taken to mean that this method 
of ranging is better than continuous tracking em- 
ployed throughout a dive, because in the discontinu- 
ous method rate control can be introduced only on 
range finder signal, while rate control can be intro- 
duced at any time during continuous tracking. This 
Howe Laboratory study applies only to the special 
case of a fast incoming diving target when the Navy 
type of regenerative range control is available. 

A laboratory study of maintaining contact on a 
moving target with a stereoscopic range finder is re- 
ported from Ohio State University. (332) This in- 
vestigation is a comparative study of the methods 
and accessory devices which can be employed with 
the range finder to help maintain stereoscopic con- 
tact on a moving target. The report discusses the 
different types of range-compensating devices which 
are or might be used with stereoscopic range finders. 
Such a device would provide an automatic adjust- 
ment of the range-measuring mechanism as the plane 
changes range, so that the residual apparent move- 
ment in the fore and aft direction is slow and the 
amount that the stereoscopic observer has to manipu- 
late his controls is reduced to a minimum. Such de- 
vices are the automatic compensation for rate of 
change in range in the Ml Height Finder and range 


{[[nisTRiCTErr 


99 


100 


MISCELLANEOUS FACTORS OF OPERATION 


compensation provided by the control knob. Other 
types of possible rate-compensating^ devices are illu.s- 
trated in the text as well as a device, used experi- 
mentally in this study, to provide automatic bracket- 
ing throughout observation of the target. A detailed 
description of the automatic bracketing apparatus 
is given in the text. This is accomplished by two ro- 
tating prisms, base up and base down which change 
the apparent position of the target as they rotate. 
Various cycle speeds and amplitudes and various 
degrees of contrast between target and background 
were employed. From the data of preliminary experi- 
ments, it can be inferred that when the target con- 
trast is low, and the amplitude of excursion is small 
in the automatic bracketing situation, there must be 
a limit below which the duration of the cycle cannot 
be reduced. But when the contrast is high and the 
amplitude of excursion is large, no explanation can 
be found in the flash exposure data for the periodic 
disappearance of fore and aft movement which oc- 
curs in automatic bracketing as the cycle frequency 
is increased, nor can an explanation be found for the 
final complete disappearance of fore and aft move- 
ment at higher frequencies. In the flash exposure 
experiment, the frequency of exposures was kept 
constantly low at one exposure every 3.96 seconds. 
Therefore the investigators believe that the interval 
between exposures is a determining factor in the 
situation in addition to the length of exposure. 

It was decided that experiments with cycle fre- 
quencies higher than 30 per minute were useless. In 
a second set of experiments this frequency was em- 
ployed as a standard. The results indicate that a 
superior performance can be obtained with inter- 
mittent rather than with continuous exposure. Not 
only is the minimum perceptible amplitude of ex- 
cursion larger in the case of continuous exposure but 
it was found that the experiment had to be discon- 
tinued at higher levels of contrast than in the case 
of intermittent exposure. Also, except at the inter- 
mediate levels of contrast, the minimum perceptible 
range of movement is consistently greater when meas- 
ured with the range decreasing than with the range 
increasing. A comparison is made of the amplitudes 
of hand and automatic bracketing necessary to pro- 
duce a just noticeable difference between target and 
reticle. It was found that the amplitudes required for 
a minimum perceptible bracketing movement are 
about the same in the case of hand bracketing as in 
automatic bracketing. In the case of normal hand 


bracketing in which the subject is permitted to move 
the target image out beyond the plane of the reticle, 
the amplitude of the bracketing excursion is much 
greater than when it is limited to points just per- 
ceptibly in front of or behind the reticle. This means 
that setting the target at supraliminal distances in 
front of or behind the target gives the subject a more 
certain feeling that he is moving the target equal 
distances to either side of the reticle. It is important 
to note that hand bracketing movements are of less 
frequent occurrence than automatic bracketing 
movements, especially at the lower levels of con- 
trast. This probably means that the automatic brack- 
eting technique was penalized, in this experiment, 
by using a higher cycle frequency than that employed 
in the hand bracketing technique. In other words, 
the experiment involving hand bracketing technique 
can be regarded as an experiment to determine the 
most satisfactory cycle frequency to use in the auto- 
matic bracketing situation. There is certainly no 
limitation to the subject’s faster manipulation of the 
knob and the frequency of bracketing movements is 
probably dependent upon the visual perception of 
the bracketing movement. The result might be 
taken, therefore, as evidence that if automatic brack- 
eting were employed in connection with a range 
finder, the cycle frequency should be somewhat 
lower than 20 per minute. Indeed, the cycle fre- 
quency probably should be variable to correspond 
to different degrees of visibility of the target. 

Another section of this report has to do with range 
measurements on a stationary target with automatic 
bracketing. A cycle frequency of 30 per minute was 
employed. The results indicated no appreciable dif- 
ference with and without automatic bracketing. Still 
another section deals with the detection of slow drifts, 
using continuous contact and automatic bracketing. 
No considerable differences were detected between 
the two techniques although automatic bracketing 
gave slightly better results than continuous contact. 
The records correspond closely in shape to the true 
course of the target but show a certain amount of 
lag and also a certain amount of lateral displacement, 
which indicates a constant error in the range meas- 
urements. 

The investigators conclude that if automatic 
bracketing is used at all, it ought to be used in con- 
junction with intermittent exposures. The maxi- 
mum cycle frequency that can be employed with such 
a device is about 30 per minute or even somewhat 


RESTRICTED' 
— => 



FOCUSSING OF TELESCOPIC EYEPIECE 


lOI 


lower. The amplitude of excursion with the auto- 
matic bracketing should be variable depending upon 
the visibility of the target but for a high contrast 
target the minimum excursion should be at least 
60 seconds. If the frequency is made greater or the 
amplitude smaller, the apparent fore and aft move- 
ment tends to cease periodically or completely after 
one fixates the oscillating target for a period of time. 
The broken contact method, if used at all, has to be 
used in conjunction with an interpreter-controlled 
compensating device, and hence if an automatic or 
observer-controlled rate compensating device is used, 
either continuous contact or automatic bracketing 
would have to be employed. The continuous contact 
technique, automatic bracketing, and the broken 
contact techniques have been compared with respect 
to their capacity for detecting slow drifts. The data 
relating to lag, departure from true range, precision 
and the like do not show that any one of these 
methods is markedly superior to the other two. 

Two field studies are reported by the Princeton 
Laboratory at Fort Monroe on this general topic. 
The first of these has to do with the distribution of 
range finder readings when different methods of con- 
tact are used. (392) The data is from five observers 
on one course with fixed target. Last motion increas- 
ing and last motion decreasing were employed. The 
distribution of readings was not significantly skewed 
and the histogram was flat topped for the last motion 
increasing series. For last motion decreasing, the 
histogram was significantly skewed toward the shorter 
range and was significantly peaked in form. Similar 
results were found for the second Princeton Labora- 
tory study at Fort Monroe. (422) 

A laboratory study was made at the Howe Labora- 
tory comparing the accuracy of stereoscopic ranging 
on different types of targets. (312) Twenty totally 
untrained subjects and six observers previously 
highly trained only on difficult moving targets were 
tested with five different types of targets. Fixed tar- 
gets were employed and simulated dives at 300 knots 
with 0, 150 and 250 knots regeneration. A simplified 
stereoscopic training instrument was used which is 
described in an appendix of the report. It was found 
that the average performance of all the observers 
trained from 3 to 6 months previously on difficult 
targets is about 0.5 to 9 UOE better on all types of 
targets than is the average performance of all the 
untrained observers. These results demonstrate that 
training on difficult targets is immediately transfer- 


able to easy targets, moving or fixed, even though 
long intervals of total lack of practice intervene. For 
one continuous tracking series the worst trained ob- 
server is in eighth place of the 28 observers with an 
average error of 8.5 UOE while the range of error 
for all subjects runs from 3.9 to 42.8 UOE. In an- 
other series employing make-and-break contact, the 
worst trained observer is in tenth place with an 
average error of 7.1 UOE while the range of error 
scores for the whole group runs from 3.1 to 40.2 UOE. 
In both series, previously trained observers are in the 
first four places. 

12 2 FOCUSSING OF TELESCOPIC 
EYE PIECES 

At Tufts College, a study of diopter settings of the 
eyepieces was made with a Navy Mark 2 Trainer. 
(555) Stereoscopic ranging was made on a stationary 
target. After preliminary instruction and trials on 
the instrument, 19 N.R.O.T.C. subjects were given 
ten trials of 20 stereo settings each of a stationary 
target. For five of the trials, the right and left eye 
pieces of the instrument were set at zero on the 
diopter scale by the experimenter; for the other five 
the subject set the eyepieces himself. Stereoscopic 
performance, for these untrained observers, was not 
found to be significantly different under these two 
conditions of diopter setting. During further trials 
the same subjects made 40 settings of the eyepieces 
and 30 stereo settings on a fixed target. Only the 
variability of the settings was found to be signifi- 
cantly correlated with either the average (r = 0.70) 
or the variability (r = 0.57) of the diopter settings. 
A reliability coefficient for diopter settings was found 
to be 0.81, showing consistency in the observers’ 
capacity to set the oculars by himself. It will be noted 
that all of these subjects were untrained, that no state- 
ment of eye conditions is noted and that all are mem- 
bers of the N.R.O.T.C., which means that they were 
carefully selected for visual defects as well as for other 
factors. 

A new method of obtaining eye focus on the height 
finder is reported by the Princeton Laboratory at 
Fort Monroe. (528) The method is based upon a 
series of judgments which the observer makes of the 
clarity or lack of clarity of the reticle pattern. The 
method consists of five steps. (1) The correct inter- 
pupillary distance is set into the eyepiece assembly. 
(2) The observer sets both eyepieces to 2 diopters. 


Restricted ” 


102 


MISCELLANEOUS FACTORS OF OPERATION 


(3) He observes biiiocularly ihe reticle pattern 
against a sky background. (4) This pattern is judged 
to be clear or not clear. If the former, the focus is 
used; if the latter, the setting for each eyepiece is 
reduced to -|-1.75 diopters, and the man views the 
field again. In resetting the eyepieces he takes his 
eyes from the instrument and sets each piece inde- 
pendently, making sure that the setting is accurate. 
(5) This procedure of resetting the eyepieces for suc- 
cessive observations is repeated, decreasing the set- 
ting by 0.25 diopter, until, for the first time, the 
observer sees the reticles as clear. The method is 
characterized as a binocular focusing method which 
establishes an equal eye focus for both eyes, and 
which determines this focus correct to the nearest 
quarter diopter. 

A discussion follows as to why this method is pre- 
ferred to the other two possible methods, namely, 
focusing each eyepiece separately and possibly to 
different amounts by bracketing to the clearest posi- 
tion for each eye, or rotating each eyepiece separately 
from maximum plus to the point where the reticle 
appears clear monocularly. Unnecessary minus in the 
setting is automatically compensated for by addi- 
tional accommodation effort, making it impossible 
for the observer to tell that the minus is excessive and 
hence leading to eyestrain for continued observation. 
Convergence as well as accommodation is stimulated 
by excessive minus. For the 26 men in a Stereoscopic 
Observers’ Class at Fort Monroe the correlation be- 
tween the binocular focus data and refractions was 
compared with the correlation between the custom- 
ary focus for right and left eye and monocular re- 
fractive data, and these were found to be very similar: 
-|-0.52 binocular; +0.52 used for right eye and +0.44 
used for left eye. More important is the fact that the 
absolute values of the binocular settings correspond 
much more nearly to the absolute values of the re- 
fractive data than do the focus settings regularly 
used by the men. Three trials in immediate succes- 
sion were made focusing by the binocular method. 
In only three cases out of 29 the difference between 
the greatest plus and the greatest minus focus ex- 
ceeded 0.25 diopters. In two cases the range of these 
focuses was 0.50 diopter and in the other 0.75 diopter. 

A study by Tufts College on the subject of diopter 
settings was made at the Advanced Fire Control 
School at the Washington Navy Yard. (574) This 
study was to determine the relationship of diopter 
settings of the eyepieces to subsequent scores of 
stereoscopic ranging on a stationary target on the 


Mark 2 Navy Trainer. An investigation was made 
with 82 trainees who had no previous experience 
with stereoscopic instruments. The ultimate pur- 
pose of the experiment was to determine whether 
the relationship is close enough to warrant using 
knowledge of diopter settings to select stereoscopic 
range finder operators. Diopter setting scores were 
correlated with average and variability scores of 
stereoscopic performance. The correlation coeffici- 
ents computed were low (0.008 to 0.303) and may be 
regarded as significantly greater than zero only in 
the instance comparing variability of stereo per- 
formance and the difference between average diopter 
settings of the two eyes. 

123 height OF IMAGE ADJUSTMENT 

A report from the Applied Psychology Panel, 
NDRC, dealing with height of image adjustment on 
the stereoscopic height finder, (74) records the re- 
sults of several short experiments on the precision 
of height of image adjustments. It was found (I) that 
the average error of the height of image adjustment 
is about 0.5 mil of apparent field; (2) that the monoc- 
ular and double image methods of adjustment are 
equally efficient for daytime observation; (3) that the 
precision of height of image adjustments does not 
change with the introduction of end window stops; 

(4) that height of image adjustment errors of 3 mils 
have a significant effect on the precision of RCS read- 
ings on stars but no effect on the precision of range 
readings taken in the daytime on ground targets. 
Finally, good observers are, in general, more skilled 
at making height of image adjustments than are poor 
stereoscopic observers. From these findings the fol- 
lowing recommendations are made. 

1. Both the monocular and binocular methods of 
making height of image adjustment should be taught 
all height finder and range finder operators. 

2. Operators should make height of image adjust- 
ments with end window stops on the instrument 
when the instrument is to be used with stops for 
range or height readings. 

3. Height finder and range finder operators should 
be given personal instruction on, and be required to 
record, height of image adjustment in order to im- 
prove precision in this adjustment. 

A Brown University report in regard to the rela- 
tion of the height of image adjustment error as found 
for different sorts of reticles will be found described 
in the section on reticle design. (179) 




RESTRICTEI 


3 


THE HEIGHT FINDER AS A SPOTTING INSTRUMENT 


103 


Two British reports deal with the problem of 
height of image error in stereoscopic range finders 
as reported by HMS Excellent. (304, 305) In the first 
report, experimental data show that when height of 
image error is present to the extent of about 10 min- 
utes of arc of apparent field, an increase in mean 
consistency of the order of 60 per cent may be ex- 
pected. The results also suggest that 3.5 minutes of 
arc is the amount of error which, in an Ml Stereo- 
scopic Height Finder, just begins to affect per- 
formance. 

Concurrently with this work, and presented in the 
second report, trials were made to determine the 
accuracy with which men can take out height of 
image adjustment error by the various available 
methods. The results indicate that men with average 
training could not take out this error accurately 
enough even by the monocular method. This was 
true even after they had had extensive practice. The 
investigators describe a new prism attachment, which 
in the future could be made an integral part of the 
range finder, by which the height of image error can 
be removed with great accuracy— to at least 0.05 to 
0.25 mils or better. 

Finally the report emphasizes the great importance 
of accurate adjustment of the stereoscopic range 
finders for height of image. This will necessitate very 
careful training at gunnery schools and constant 
practice at sea, and operators must make this adjust- 
ment with care on closing into combat. If the prism 
attachment referred to comes up to the high standard 
expected of it, and is incorporated in the range 
finder, it is anticipated the adjustment will be easily 
made and the training problem much simplified. 

12.4 the HEIGHT FINDER AS A 
SPOTTING INSTRUMENT 

A laboratory study was made at the Ohio State 
University to determine the speed and accuracy in 


spotting with this sort of stereoscopic instrument. 
(330) The apparatus and method used in the investi- 
gation are described in detail in this report. The 
results indicate that under laboratory conditions in 
which a target of high contrast and sharp borders is 
used, subjects have been found to render stereoscopic 
judgments of the position of the target relative to a 
reticle mark after a lapse of time ranging from 0.4 
to 1.5 seconds following the onset of the exposure. 
When the target is seen to lie in the same plane as 
the reticle, a longer time is required on the average 
to make the judgment than when the target is seen 
at a perceptible distance in front of or behind the 
reticle. These data refer to the situation in which the 
subject is instructed to respond as quickly as he can. 
If the subject is instructed to delay his response until 
he feels certain that further prolongation of the ex- 
posure would not influence his accuracy, it is found 
that some subjects show only slightly longer reaction 
times than in the situation in which they react as 
quickly as possible, but on the other extreme, one 
subject was found to use about three times as much 
time before making judgment. Subjects who used a 
longer time to make a response under these circum- 
stances also showed a marked improvement in accu- 
racy and precision. One subject showed no such im- 
provement even when required to delay the judg- 
ment for five seconds. He was the fastest and most 
highly trained of the subjects and the reactions of the 
rest are probably more typical of Service personnel. 
Hence stereoscopic observers who are assigned to the 
duty of spotting should be encouraged to prolong 
the observation of the cloud of smoke produced by 
the bursting shell for approximately three seconds 
before making a judgment, provided the rate of fire is 
such that a delay of this length can be tolerated. Re- 
ducing the contrast slightly increases the reaction 
time for the stereoscopic judgment as well as decreas- 
ing accuracy and precision. Decreasing target size 
also leads to an increase in reaction time. 


RESTRICTED 



PART III 


THE HUMAN OPERATOR 


I T HAS long been known that there is a very small 
percentage of human beings who can operate a 
range finder, especially a stereoscopic instrument, 
with anything approaching maximal accuracy. Chap- 
ters 13 and 14 outline the attempts to provide the 
Services with the best possible operators by selection 
of those most apt to become efficient operators and by 
the subsequent training of those selected. 

Chapter 13 reports the experimental work on 
selection. The instruments themselves set certain 
limitations, such as a maximal and minimal range 
of interocular settings, or height limitation of the 
operator for the Army instruments. Furthermore, 
good intelligence and good mechanical ability are 
required. But more important still is the require- 
ment of excellent stereoscopic acuity. At the begin- 
ning of the present war, no satisfactory and adequate 
test of stereoscopic acuity existed which would meas- 
ure this factor with sufficient accuracy to be used as 
a selection device. A considerable part of this chapter 
deals with the search for such a stereoscopic acuity 
test. Many devices were tried and finally the stereo- 
vertical test was found to give satisfactory results. 
The chapter also tells of the battery of selection tests 
for stereoscopic range finder operators which were 
adopted by both Services and the plans and experi- 
ence of the administration of these selection pro- 
cedures in the Services. Validation of this selection 
battery of tests indicates that they adequately skim 


off the approximately best 3 per cent of the Service 
population who would be most benefited by subse- 
quent training and who would become the most 
adequate range finder operators. 

It was early recognized that the efficiency of a 
battery must depend on the continued accuracy of 
the range finder operator under the stress of combat. 
Hence some tests for a rough determination of emo- 
tional stability seemed desirable. The development 
of a group personnel questionnaire and of its valida- 
tion is outlined in this chapter. It was never adopted 
for selection of range finder operators by either 
Service although it is widely employed in certain 
branches of the Navy and the Army where emotional 
stability is required by certain men in key positions 
or in performing especially hazardous tasks. 

Chapter 14 deals with the training of the range 
finder operators so selected. It outlines measures of 
accuracy of performance and methods for measuring 
such accuracies. Learning curves, training methods 
and training devices are discussed. In this connection 
the Eastman Trainer was developed and experiments 
showed that preliminary training could be carried 
on as adequately with this training device as with 
the actual range finders and real targets. Thus many 
more trained operators could be made ready for com- 
bat at a time when the number of range and height 
finders available for training purposes was limited 
and aerial missions difficult to obtain. 


RESTRfCTE 


105 




Chapter 13 

SELECTION OF RANGE FINDER OPERATORS 


13.1 the battery of selection 
TESTS AND THEIR VALIDATION 

O NE OF the first problems attacked by the Prince- 
ton Laboratory at Fort Monroe was an attempt 
to analyze the special human competencies and abil- 
ities which would make an individual a good range 
finder observer. If this could be accomplished, it 
would then be simple to set up a series of tests for the 
selection of such operators. Previous to this time both 
the Army and Navy selected range finder operators 
largely by employing preliminary training as a 
screening procedure, so far as the determination of 
stereoscopic acuities were concerned. Obviously such 
a method was too slow and too cumbersome to be 
used in war time and with a very rapidly expanding 
Service personnel. 

7'he Princeton Laboratory staff contained oph- 
thalmologists, psychologists, and psychophysiologists 
who tested the Height Finder School personnel with 
a large number of tests, each of which seemed to have 
high face validity, in an effort to determine which of 
these might have the requisite internal consistency 
and which might actually validate against the School 
scores. 

The Princeton Laboratory and several other 
groups tried a number of tests, all of which had a 
certain amount of face validity, to discover proced- 
ures which might aid in the selection of such per- 
sonnel. The selection of stereoscopic range finder 
observers was a new problem and hence any test was 
tried which indicated a reasonable amount of face 
validity. Many of these tests proved of value and were 
adopted by the Services. For these particular scores 
or screening limits had to be set on the basis of 
experimental experience. Many of the procedures 
proved of no value and were dropped. Both adopted 
and rejected procedures are given later in this section. 

Very early in 1942, just after Pearl Harbor, the 
Service Liaison Officers to the Princeton Laboratory 
project strongly urged that Section D-2 recommend 
a selection battery of tests for range finder operators. 
It was pointed out that the recommendation of such 
a battery in the then present state of knowledge could 
be little more than an informed guess, inasmuch as 


proper validation on a sufficient number of cases 
had not been made up to that time. However, be- 
cause the urgency of the situation was evident, 
Section D-2 made tentative suggestion of such a bat- 
tery of selection tests. (4) 

Minimum Standards 

This report suggests minimiun standards as 
follows: 

1. General Intelligence: A score of at least 100 on 
Army General classification lA or at least 85 on Navy 
O’Rourke General Classification Test. 

2. Height: Not less than 5 feet 6 inches. (This 
requirement could be disregarded for Navy person- 
nel due to difference in equipment. It is necessary in 
the Army to permit the operator to look into the 
oculars when ranging near the horizon.) 

3. Mechanical Comprehension: A grade of I, II, or 
III on Part 3 of Army test MA2 or MA3, or at least 
75 on Navy O’Rourke Mechanical Comprehension 
Test. 

4. Vision: Visual acuity of at least 20/20 in each 

eye 

Hyperphoria not greater than I/2 prism 
diopter 

Exophoria not greater than 6 prism 
diopters 

Esophoria not greater than 6 prism 
diopters 

5. Interpupillary Distance: Not less than 60 nor 
greater than 70 millimeters. (The interpupillary 
scale on the instruments actually runs from 58 to 
72 mm but measurement of this distance was so fre- 
quently unreliable that the narrower distance was 
recommended.) 

6. Desire for Training: After the nature and im- 
portance of the duties have been explained, select 
only those who answer “Yes” to the question, “Do 
you wish to be trained as a stereoscopic observer?” 
(This qualification was added to improve the mo- 
tivation of the group of trainees.) 

7. Projection Eikonometer: A grade of I or II 
(This is a test of stereoscopic acuity developed for 
the project.) 


RFSTRICl’FD 


107 



108 


SELECTION OF RANGE FINDER OPERATORS 


8. Modified M2 Trainer: A grade of I or II on the 
Stereoscopic Trainer M2, modified to include power- 
driven change in range. 

It should be pointed out that the Eikonometer 
and M2 Trainer tests are the only two new tests over 
and above those already administered as routine in 
the Services. 

These recommendations were promptly adopted 
by the Army and put into effect as soon as possible, 
initially under the supervision of the staff of the 
Princeton Laboratory. A description of these and 
other rejected tests will be found later in this section. 

This first report was followed less than 8 months 
later by another which recommended the instru- 
ments to be employed in such an examination. (10) 
(covering 354). This report emphasized the two new 
instruments, namely, the Eikonometer and the modi- 
fied M2 Trainer. 


Screening Experience 

A few weeks later this was followed by a report 
(11) (covering 355) giving a detailed manual for the 
administration of these tests. Some six weeks later 
and on the basis of a background of practical experi- 
ence in the administration of these tests, a supple- 
mental manual was recommended. (25) (covering 
356). The changes were made on the basis of valida- 
tion on the classes of stereoscopic observers at Fort 
Monroe and on the testing of large numbers of re- 
cruits at Fort Eustis and Camp Wallace. This report 
outlines the organization of a testing center. The 
results of experience indicate that only 3 per cent of 
a normal Service population succeed in passing all 
of the screening tests. However 80 per cent can be 
eliminated on the basis of a study of the Service 
Qualification card or the basis of General Classifica- 
tion Test or Mechanical Aptitude Test scores or 
because of height or age. Visual tests eliminate a 
further 8 per cent which leaves only 12 per cent of 
the total population who need to be tested on the 
Eikonometer and M2 Trainer. Certain substitutions 
of testing material are also recommended: a revised 
visual acuity chart with the Luckish-Moss Illumi- 
nator and a new chart to test heterophoria. The 
adoption of the Modified Massachusetts Vision Test 
Kit is recommended. Detailed instructions for the 
form of the Eikonometer Test, which is called the 
Stereo-Vertical Test, are given. By means of the de- 


velopment of a multiple recording device, this report 
indicates that it was now possible to test six subjects 
simultaneously instead of the single examination 
previously necessary. Detailed instructions are given 
for the filling out of the record cards. 

A report on the validation of the tests by the 
Princeton Laboratory on students at the Height 
Finder School at Fort Monroe is given extensive 
treatment. (357) Unfortunately for this work, the 
primary mission of the School which was to train the 
largest number of best height finder operators, was 
so urgent that it could not be modified. As a result 
the population available was far too homogeneous 
to form a basis for validation and also the numbers 
were too small. In addition, many of the men report- 
ing to the School had had previous ranging experi- 
ence and this resulted in the individuals starting with 
widely different degrees of previous experience. A 
total of 179 men in six classes form the basis of the 
study. Men of the first five classes were assigned to the 
School from active organizations on the recom- 
mendation of their battery commanders. The men 
of the last class were selected by the Princeton Field 
Laboratory staff at Fort Eustis. Many tests were given 
to these men and only the few already specified above 
gave promise. Others will be discussed later in this 
section. The performance criterion employed was 
the observer’s UOE which, it is believed, was an in- 
complete measure of proficiency but which was the 
criterion employed by the School for graduation. 
Observers’ UOE were taken on fixed target and 
aerial target performance and grades on theory and 
records were also available. Only 5 men failed in 
theory and records, while 15 failed to pass the cri- 
terion of UOE on fixed targets, and 34 failed to pass 
the criterion of UOE for aerial targets. 

The results form the basis for the recommendation 
of the Stereo-Vertical test rather than other possible 
tests with the Eikonometer. There was a positive cor- 
relation between mental ability and records and 
theory grade as well as a positive correlation between 
tests of mental ability and General Classification 
Test scores. 

Standardization scores were obtained from testing 
from 223 to 291 men of the 5th Training Battalion 
at Fort Eustis who had an Army General Classifica- 
tion Test lA score of over 90. In this way cutoff 
scores for Mechanical Comprehension, Eikonometer 
and the M2 Trainer were obtained. A revised battery 
of tests was given subsequently to 492 men at Camp 



VALIDATION OF PRINCETON LABORATORY TEST SET 


109 


Eiistis. Of these, 169 were given both the Eikono- 
meter and M2 Trainer tests. Questions of operator 
differences were examined and standardized testing 
procedures were worked out. 

At this point of development work on selection 
was turned over to the NRG Committee on Service 
Personnel— Selection and Training, which was sub- 
sequently absorbed into the NDRC Applied Psy- 
chology Panel. These groups retained much of the 
Princeton Laboratory personnel working on this 
problem and continued the validation of this screen- 
ing procedure. Work was continued at the Height 
Finder School at Camp Davis. The net result of this 
work was the recommendation (73) to eliminate the 
M2 Trainer test because the Projection Eikonometer 
test alone gave more valid results and to lower 
slightly the passing score for the Eikonometer test. 
However, the substitution of the M2 Trainer test in 
place of the Eikonometer test is recommended for 
field or other situations where the Eikonometer is 
not available. These recommendations were subse- 
quently adopted by the Navy for stereoscopic range 
finder operators. Results of the screening of 37,500 
men at Fort Eustis during the year of June 1, 1942 
to May 31, 1943 show that only 1,474 or slightly over 
4 per cent of the population were accepted. 

The validation of the tests as given in this report 
is based primarily on three classes at the Height 
Finder School at Camp Davis. These men met all 
the qualifications noted above except that a certain 
proportion failed to meet the scores for the M2 
Trainer and the Eikonometer, but the group was 
representative of a random sample of the Army pop- 
ulation in this respect. Previously the urgent need 
for trained stereoscopic observers of the highest 
possible quality made such a procedure impossible. 
Besides the Stereo-Vertical test with the Eikonometer 
and the M2 Trainer test, the Vectograph Pursuit and 
the Dearborn- Johnston tests were given these candi- 
dates. For criteria the following five measures of 
performance on the height finder were used as vali- 
dation; (1) variability score in UOE; (2) course 
error score in UOE; (3) the “hit” score in per cent; 
(4) percentage of “good courses” score and (5) the 
sum of the course error and the variability scores in 
UOE. Criteria 1, 2 and 5 are “error” scores while 3 
and 4 are “hit” scores. The variability score at a 
passing grade of 6 UOE and below had been used 
as the conventional measure of performance then in 
use for graduation at the Height Finder School. The 


new cutoff scores for the test of stereopsis are realisti- 
cally based upon a consideration of a balance be- 
tween obtaining a sufficient number of men for train- 
ing, on the one hand, and of instruction cost, on the 
other. The Dearborn- Johnston test was not found 
satisfactory for purposes of selection. The Vecto- 
graph Pursuit Test might be considered advantage- 
ously as an alternative to the Projection Eikonometer 
test but was not recommended because of the need 
for further development and because of procure- 
ment problems. The results also indicate that some 
of the single tests did about as well as combinations 
of stereoscopic tests in terms of both selection and 
instruction costs. Therefore, the Stereo-Vertical test 
was recommended above because the Eikonometer 
in a satisfactorily developed form was available at 
testing centers with the modified M2 Trainer as a 
stand-by testing instrument. 

On June 1, 1942 an actual testing center was set 
up at Fort Eustis staffed by Princeton Laboratory 
and by Service personnel. This field laboratory had 
a dual purpose: (1) to aid the Army in selection of 
stereoscopic operator trainees, and (2) to train Army 
personnel in the selection techniques employed so 
that aid from the Princeton Laboratory would no 
longer be required. Subsequently a similar testing 
center was set up at Camp Wallace on September 1, 
1942, with the assistance of the Princeton Laboratory. 
The results of the testing in these two centers are 
recorded in a number of reports. (370) These form 
the basis of knowledge of selection costs from an 
unselected Service sample. These reports also contain 
a great deal of information regarding modification 
and standardization of the several tests in the battery 
necessitated by the shift from laboratory to field con- 
ditions. There will also be found a day-to-day diary 
which is of some general interest as indicating the 
difficulties attendant on setting up such a new proj- 
ect in a Service organization, in spite of the splendid 
cooperation given by the Army. 

In these reports will be found the selection costs 
for each sub-test in the battery. A single sample indi- 
cates the selection costs. The results are those of the 
men tested between June 10 and June 30, 1942. 
Approximately 4,500 Army records were studied and, 
after elimination because of GCT and MA test scores, 
height, or indication that the men did not want 
training as stereoscopic observers, 687 men were 
selected for examination at the Stereoscopic Testing 
Center. The table on the following page indicates 


RESTRICTED 




110 


SELECTION OF RANGE FINDER OPERATORS 


the number selected and the causes of elimination. 


Total number of men tested . . 687 
Passed and recommended .... 64 

Failed Vision Tests 245 

Interpupillary distance 36 

Visual acuity 194 

Heterophoria 15 

Failed Performance Tests . . . .378 

Did not see “stereo’' 51 

Projection Eikonometer only ... 69 

M2 Trainer only 57 

Both 201 


From these figures it can readily be seen that visual 
acuity and the Eikonometer and/or M2 Trainer tests 
eliminated the largest number of men. The results 
clearly show the need for some special test of stereo- 
scopic acuity, inasmuch as 378 of the men tested 
failed on these tests after they had successfully passed 
the general vision tests. Hence the belief that “good 
round eyes” are all that are needed to make a suc- 
cessful stereoscopic range finder operator was ex- 
ploded. 

Studies were made of the relationship of the sev- 
eral tests, such as the Eikonometer and M2 Trainer. 
On the basis of such experience, standard scores were 
developed for these tests and five grades were estab- 
lished. A study was also made of the distribution of 
scores by examiners. Certain examiner differences 
were evident and therefore much greater emphasis 
was placed on administration of the tests in stand- 
ard form. 

Time studies were made which indicated that the 
average examining time per subject for the visual 
tests was 7^ minutes. The Eikonometer test required 
29.5 minutes and the M2 Trainer test required 29.4 
minutes. It should be noted that both of these tests 
required individual examination at this stage of 
development. 

A second report covering July 1-15, 1942 indicates 
that 32 men were tested daily with little variation. 
This report contains a table of organization and 
description of duties of the testing personnel. The 
staff of a single unit calls for 2 officers and 18 enlisted 
men. Plans for suitable buildings to house such a 
testing unit are given. Such an establishment should 
operate at the rate of testing approximately 2,000 
men per month. Detailed plans for testing booths 
and computing and administrative sections are given. 
There will also be found detailed descriptions of 
the necessary equipment for such a testing center. 


Finally, a course of instruction for examiner candi- 
dates, covering a 3-week period, is outlined. 

The third report covers the period July 16-31, 
1942. By this time the routine testing, computing, 
and administration of the Fort Eustis center was so 
well organized that attention was turned to training 
the additional examiners necessary to fill the cadres 
for Camp Wallace and Camp Callan, the other two 
Antiaircraft Replacement Centers. A study was made 
of the relationship between the performance of 1,053 
subjects on the Eikonometer and the M2 Trainer 
tests. The coefficient of correlation is 0.21, which is 
not significant. 

In a fourth report, covering the period August 
1-15, 1942, a comparison is made of the scores given 
by regular examiners and by student examiners. 
These show very similar distributions for the Eikono- 
meter test. For the M2 Trainer test the student ex- 
aminers gave statistically significantly lower scores. 

The fifth Eustis report covers the period August 
16-31, 1942. In this will be found the plan and results 
of a test-retest program to determine the reliability 
of the testing procedures in use, both vision and per- 
formance tests. The plan called for retest by the same 
examiner in half the cases and by a different exam- 
iner in the other half. The results indicate a reason- 
able reliability for the interocular distance measure- 
ments (91 per cent within dzl.O mm). 

For visual acuity only those who had passed were 
retested and results of the retest indicate that 97 
per cent remained in the “passing” category. Similar 
reliability was found for the tests for phoria with 
none of the men who previously passed eliminated 
by the retest. For the performance tests, it was ar- 
ranged that half of the men be tested first with the 
Eikonometer and the other half with the M2 Trainer. 
The results indicate that it makes no difference which 
test is given first. Similar test-retest distributions were 
found for the Eikonometer. There were, however, 
marked differences between distributions of the 
scores on the test and the retest on the M2 Trainer 
for all examiners. The results show considerable 
improvement in the scores obtained on the retest 
with this instrument. 

A statistical study of these results indicates that 
the reliability of this test as then administered is only 
fair, the reliability coefficient being 0.74. The relia- 
bility coefficient is improved to 0.81 when the same 
examiner makes both tests. For the M2 Trainer test 
the reliability was not good, being 0.60 or only 20 


TT^STRICT ItTI 


FORT EUSTIS TESTING CENTER RESULTS 


111 


per cent better than chance. Furthermore the relia- 
bility coefficient was not increased when both tests 
were made by the same examiner. A new method of 
scoring both performance tests failed to change the 
reliability coefficients to any considerable degree. 

One highly significant development detailed in 
this report is the development of a Multiple Projec- 
tion Eikonometer. By means of this instrument, it is 
possible to test six men simultaneously instead of 
the individual examination required for the former 
instrument. This development is of importance 
when it is remembered that the former instrument 
required approximately 25 minutes of examiner time 
for each subject tested. A manual for the operation 
of this instrument is included as an appendix to the 
report, with detailed instructions for the testing 
procedure and for computing and scoring the results. 

The sixth Eustis report covers the period of the 
month of October 1942. This report contains a re- 
vised 3-week training program for examiners, con- 
sisting of lectures, classes, seminars, examinations 
and practical testing experience. Outlines of the ten 
lectures and of the final examination are given. 

The seventh report outlines the revised organiza- 
tion of a unit consisting of 2 officers and 18 enlisted 
men who should each month be able to screen 3,500 
records, give preliminary examinations to 1,500 men 
and administer complete stereoscopic tests to 1,000 
men. Tables of organization and detailed duties of 
each of the personnel are outlined. Examination, 
computation, and clerical procedures are indicated. 
As illustrative material, a full report is given of the 
screening and examination of a typical battery 
(Battery A, 6th Battalion at Fort Eustis). 

The eighth report covers the period of the month 
of September 1942. During this period 956 men were 
tested. They were selected from an initial screening 
of 4,832 records. Of these, 155 or 16 per cent of those 
tested passed all tests. It should again be pointed out 
that only slightly more than 3 per cent of the men 
whose records were screened passed all of the tests 
and could be recommended for training as a stereo- 
scopic observer. Work was continued with a com- 
parison of the Eikonometer and the M2 Trainer as 
testing instruments. 

The ninth report covers the period of the month 
of October 1942. This report indicates that the 
Multiple Projection Eikonometer was now in use as 
a testing instrument. 

During all of this time, the testing unit was 


charged with the selection of candidates for the 
Height Finder School at Fort Monroe. These records 
are full of the sort of incidents of apparatus failure 
and their correction which are bound to be encoun- 
tered in any Field Testing Center. 

A final report in this series tells of the establish- 
ment of a second Stereoscopic Testing Center at 
Camp Wallace and covers the period October 22-31, 
1942. During the first month 1,622 records were 
screened; 40 were tested and of these 35 were recom- 
mended for stereoscopic observer training. 

Late in 1942 the Committee on Service Personnel- 
Selection and Training of the National Research 
Council was formed. Inasmuch as work in the field 
of selection of stereoscopic range finder observers 
fell so completely within the province of the Com- 
mittee, all work of this sort was turned over to them 
for further development and administration. 

A report from HMS Excellent summarizes the 
British experience with selection tests for stereo- 
scopic range takers. (301) The report sets up the 
following standards for selection with indication of 
validation experiments. (1) Interpupillary distance 
between 60 and 70 mm inclusive. (2) Ophthalmic 
standards: (a) visual acuity, each eye separately, 
6/6, (b) refractive errors; total hypermetropia under 
homatropine and cocaine, +2.0 diopters of which 
not more than 0.5 diopter may be due to astigma- 
tism; esophoria, 4 prism diopters; exophoria, 3 
prism diopters; vertical phoria 1 prism diopter. (3) 
A standard score of 150 on the SA16 (75 per cent 
must be 17.75 mm or less). (4) In cases where the 
SA16 is not available men may be accepted for stereo- 
scopic range finder training if they obtain a standard 
score of 100 on the M2 Trainer. The SA16 is a 
complicated form of a rod test presumably devel- 
oped from the Howard-Dolman Test, in which the 
stereoscopic position of 4 rods is compared with a 
standard rod, which in turn may be set at varying 
distances from the observer. Instructions and stand- 
ard scores for the SA16 and the M2 Trainer tests 
are included. It is concluded that the selection tests, 
here outlined, proved a satisfactory solution to the 
problem of selection but it was hoped that a new 
selection instrument then being designed might 
produce even better results. 

The Applied Psychology Panel has prepared a 
manual for use in the selection of fire controlmen 
(0) in the Navy who are to be trained as stereoscopic 
range finder operators. (99) It concludes the direc- 


RESTR Ii 


112 


SELECTION OF RANGE FINDER OPERATORS 


tions for the organization, equipment, and procedure 
of a testing unit in a Naval Training Center or Naval 
Receiving Ship. It also includes detailed instructions 
for the administration of the various tests and for 
the reporting of the results. No tests not elsewhere 
described are introduced. The manual calls for an 
initial screening on the basis of certain qualifications 
on the General Classification, Reading, and Mechan- 
ical Aptitude tests with an age ceiling of 30 years. 
Only those men who meet all of these qualifications 
are sent to the Vision Testing Center. Here, for 
selection and classification as stereoscopic fire con- 
trolmen they must pass three more tests: with the 
Ortho-Rater for visual acuity, heterophoria, stere- 
opsis, and color perception; with the OSRD Inter- 
pupillometer for measurement of interpupillary 
distance; and with the Multiple Projection Eikono- 
meter for stereoscopic acuity. Approximately 90 men 
can be tested each day by a single Visual Testing 
Unit. 

The following sections outline the developments 
of selection procedures, both those adopted and 
those rejected. 

13 2 GENERAL FACTORS 

Tests at the Princeton Laboratory (478, 481) indi- 
cated no relationship between range finder perform- 
ance and age, weight, height (except that short 
stature may interfere with the operation of the Army 
antiaircraft instrument when ranging near the hori- 
zon), religious interest, or rural and urban birth 
and residence. 

A number of psychomotor tests were also given. 

(474, 481) These included the two-hand co-ordina- 
tion test, a reaction co-ordination test, a pursuit 
meter test, a dotting test, and a test of work decre- 
ment. No significant correlation was found for any 
of these tests and range finder performance as meas- 
ured by the operators 2 log UOE score. With a 
steadiness test, (477, 481) there was a suggestion that 
men making a poor score on the test may be poorer 
range finder operators. The data did not give suffi- 
cient promise for further investigation. A study of 
pulse rate, blood pressure, and metabolic rate indi- 
cated little or no relationship with ranging perform- 
ance. (477) There was slight indication that poor 
performance might be associated with high pulse 
rate and either high or low blood pressure. How- 
ever, the preliminary results on a few subjects did 


not give sufficient promise for further development. 

Tests of mental ability, spatial relations, and 
mechanical comprehension were also given to four 
classes at Fort Monroe. (479, 481) No significant 
correlations were found between any of these tests 
and range finder performance. The possible excep- 
tion to this general statement was the suggestion 
that the mechanical comprehension test might have 
value as a part of a test battery, although it had little 
predictive value when given alone. 

Besides the psychophysical tests and general back- 
ground indicated in the paragraph above, a number 
of pencil and paper tests were given to the Fort 
Monroe trainees. (481) These were the Wunderlich- 
Hovland Personnel Test for mental ability; the 
Likert-Quasha Minnesota Paper Form Board for 
spatial relations and the Bennett Mechanical Com- 
prehension Test. School attendance was also con- 
sidered. No significant correlations were found again 
with any of these tests and ranging performance. 

At Fort Monroe Height Finder School, some of 
the men who reported had had previous experience 
with the Army Antiaircraft Height Finder in 
amounts varying from 2 to 15 weeks, while others 
had had no previous experience whatsoever. A study 
of the men in two classes (504) indicates that the 
average performance of the men with previous ex- 
perience is somewhat better than that of the novices. 
An arithmetic test, a specially devised dial reading 
test, and a test of aircraft identification were given 
to a group at Fort Eustis in January 1942. (502, 503) 
These tests were also given to one class at the Monroe 
Height Finder School and were found to have only 
a low correlation with School grades in both cases. 

13.3 the MEASUREMENT OF 

INTEROCULAR DISTANCE 

The limits of interpupillary distance are deter- 
mined by the limits for setting of the eyepieces on 
the instruments themselves. A study was made of 
the distribution of measured interocular distances of 
6,554 white soldiers and 127 negro soldiers (145) at 
the Stereoscopic Testing Center at Fort Eustis. The 
distributions are given in both tabular and graphic 
form. The results indicate that less than 0.4 per cent 
of a total population will be excluded beyond the 
limits of 58-72 mm and less than 4 per cent beyond 
the limits of 60-70 mm. In actual practice in the 
screening of 37,500 men at Fort Eustis, only 500 men 


RESIR ICTEtT^ 


MEASUREMENT OF INTEROCULAR DISTANCE 


113 


were eliminated because they fell outside the range 
of 60-70 mm interpupillary distance. (73) 

The early method of measurement consisted of 
the use of a millimeter scale held in front of the 
subject’s eyes. This was known to be a relatively in- 
accurate method and the use of the Shuron Pupil- 
lometer was early recommended. This instrument is 
small and easy to obtain. The reliability of measure- 
ments by such an instrument was reported for test- 
retest data for 104 men and by four examiners. (486) 
Examiner differences were apparent. The probable 
error of measurement was slightly less than 0.5 mm. 
Of the 104 men measured, 62 measurements were 
the same in the retest while 9 were either 2 mm longer 
or shorter than the original reading. 

It was pointed out (18) that serious range errors 
can occur unless the distance between the oculars of 
the range finder accurately corresponds to the inter- 
ocular distance of the observer. At this time it was 
recommended that an accuracy requirement of 0.25 
mm be placed upon the interocular adjustment 
mechanism. At the same time it was noted that no 
entirely satisfactory means for measuring the ob- 
server’s interpupillary distance is now available. It 
was obvious that existing methods of measurement 
were not sufficiently accurate or reliable for this 
criterion. 

Intensive research at Harvard University (282) 
following the suggestion from the Princeton Labora- 
tory (522) indicated that the lines of sight in the 
telescopes of the range finders move with the oculars, 
whereas the observer’s eyes do not. Hence, the binoc- 
ular parallax between target and reticle may be 
changed solely by varying the separation between 
the eyepieces. This problem is discussed in full in 
Section IV B. 

Study was made of the several existing instruments 
for measurements of this type. The Zeiss Gauge, and 
the Shuron Gauge were found to be unsatisfactory, 
primarily because of parallax introduced by IPD 
between the eyes of the subject and of the examiner. 

This error can be eliminated by introducing a 
mirror with the Shuron instrument so that the sub- 
ject makes his own measurements. Such a system was 
introduced in the Bausch &: Lomb Gauge, which was 
also tried. 

The Harvard group then developed an IPD gauge 
consisting of a mirror, two stadia wires, and a vernier 
attachment for changing the separation of the wires. 
The subject changed the separation until the wires 


and their mirrored images were coincident. Tests of 
five experienced observers, each of whom took five 
readings every day for 25 consecutive days, give aver- 
age mean variation of the IPD measurement range 
from 0.07 mm to 0.11 mm. The mean variation of 
the averages varied from 0.03 mm to 0.11 mm. 

During these experiments, however, the observers 
frequently reported parallax between the stadia 
wires and their reflected images. The reports were 
noteworthy because the parallax was perceived when 
the real wires appeared to bisect the observer’s pupils. 
This state of affairs suggested that the observer’s 
external lines of sight did not pierce the geometrical 
centers of their natural pupils. If this were true, it 
meant that IPD is an inaccurate measure of the dis- 
tance separating corresponding lines of sight of the 
two eyes. Hence the Harvard group proceeded to 
measure the separation between the stadia wires 
when the parallax between each wire and its reflected 
image had been reduced to zero. Such measurements 
were called interaxial distance [lAD] and were 
found to have as high accuracy and reliability as had 
been found for IPD. 

For the five skilled observers lAD and IPD were 
found to differ in amounts varying from 0.06 to 0.77 
mm and in four of the five cases the lAD was larger 
than the IPD. Fourteen naive individuals were tested 
with the following results: in ten cases the lAD ex- 
ceeded the IPD; in two cases it was smaller; and in 
two cases the differences were insignificant. Similar 
results were obtained with Service personnel in a 
Coast Artillery Battery and at Camp Davis. In both 
samples it was found that there was only a small per- 
centage who had equal IPD and lAD and that in 
most cases the lAD measurement was greater than 
that of IPD. 

An engineered model of the interaxialometer was 
constructed under Navy auspices and was tested with 
both trained observers and with Service personnel. 
It proved to be both accurate and reliable, with a 
very large percentage of the measurements having 
an average mean variation of not more than 0.15 mm. 
Considerable experimentation was done to deter- 
mine the properties of lAD as affected by accommo- 
dated distance, ocular vergence, uncontrolled verg- 
ence, lateral eye movements, vertical rotary eye 
movements, and vertical rotary head movements. 

A study with range finders was made at Bausch Sc 
Lomb (107) to determine if the IPD or the lAD 
should be set into the instruments. Readings were 


RESTPJr 


114 


SELECTION OF RANGE FINDER OPERATORS 


taken on an indoor collimator with various separa- 
tions of the eyepieces with expert observers whose 
I PD and I AD had been determined. Of the four 
observers, three turned out to have I AD narrower 
than IPD. For two observers the best results are ob- 
tained with the IPD set into the instrument, for one 
observer with the lAD measurement, and for the 
fourth observer with neither. Results from Camp 
Davis favored the use of the IPD although it was 
again demonstrated that considerable differences 
were apparent between the IPD and I AD of most 
of the men (150). 

The results are somewhat confused. The Harvard 
group concluded that the oculars should be set at 
the interaxial when using large exit pupils and the 
interpupillary distance when using small ones (the 
latter because the danger of “clipping” is thereby 
minimized). It was evident, however, that when the 
exit pupils are very small (ca. 1 mm) the errors due 
to incorrect interocular setting are negligible. With 
large exit pupils the interocular setting is critical, 
and, in the case of some observers at least, equal to 
the interaxial separation. 

Because it seemed inadvisable to have two forms 
of interocular measurement to be set into the instru- 
ment at different times and under different condi- 
tions and to emphasize the value of stopping down 
the range finder whenever possible (cf. section on 
perspective errors), it was recommended (66) that 
the IPD be adopted throughout and that the instru- 
ments be used in a stopped down condition whenever 
possible. It was also recommended that the IPD be 
measured by means of the Bausch Sc Tomb Duplex 
PD Gauge. 

An instrument was developed at Camp Davis (147) 
which combined a Shuron Interpupillometer with 
a mirrored reflection. This instrument measures IPD. 
For 36 subjects the difference between the median 
value of two sets of three readings each was the same 
in 13 cases; varied by 0.25 mm in 15 cases; by 0.5 mm 
in 7 cases and by 0.75 mm in the remaining case. 
Test-retest reliability was high with no difference in 
36 per cent of the cases; 0.25 mm difference in 42 per 
cent; 0.50 mm difference in 19 per cent and 0.75 mm 
difference in the remaining 3 per cent. 

A comparative test of the reliability and precision 
of the NDRC and the Bausch and Lomb Interpupil- 
lometers was performed at Fort Lauderdale. (82) A 
total of 81 men were measured twice by each instru- 
ment to determine their interpupillary distance. 


This analysis shows that the range of measures of 
interpupillary distance obtained was comparable to 
that of a male Army population and also that there 
are no significant differences between the means and 
standard deviations of the measures obtained on the 
two instruments. However it was found that the 
dispersion of differences between test and retest is 
significantly less for the NDRC instrument than for 
the B & L Duplex P-D Gauge. A significantly greater 
proportion of measurements obtained with the 
NDRC instrument fall within the tolerable limits 
required for range finder operation (±0.25 mm) 
than those obtained with the B &: L instrument. It 
was also found that the coefficients of correlation for 
reliability of both instruments are very high but 
those for the NDRC instrument are significantly 
higher, and that the probable error of the B & L 
instrument is greater than the NDRC. On the basis 
of these findings, the use of the NDRC Interpupil- 
lometer was recommended to the Services. A descrip- 
tion of the instrument and a manual for its use are 
included as an appendix to the report. 

A complete description of the laboratory model 
of the NDRC Interpupillometer and its carrying 
case are outlined in a report (106). Suggestions for 
modification of this laboratory model for a field 
instrument are also given. The instrument consists 
of two eyepieces whose horizontal separation is con- 
trolled by adjustment screws. Bulbs below the eye- 
pieces illuminate the pupils of the individual whose 
interpupillary distance is being measured. A mirror 
behind the eyepieces provides a reflection of the 
pupils and the hair line which is etched vertically 
in the glass of each eyepiece. A vernier millimeter 
scale which can be read from the rear of the instru- 
ment indicates the amount of separation of the hair- 
lines. An adjustment is provided in the instrument 
and a method of calibration is recommended in this 
report which assures that the scale correctly indicates 
the separation of the two hairlines. 

A new measure of interpupillary distance is re- 
ported by Brown University. (180) The technique in- 
volves the photographing of the two eyes simultane- 
ously with an adapted form of graflex stereoscopic 
camera against a millimeter scale. The report gives 
the results of 37 subjects whose interpupillary dis- 
tance was measured twice each day by this new device 
and the NDRC Interpupillometer with a week elaps- 
ingbetween the two sets of measurements. It was found 
that the distributions of IPD found by the two 



INTERPUPILLOMETERS-VISUAL TESTING 


115 


methods were very similar. The reliabilities of the 
two methods, as judged by differences between test 
and retest of each subject, are not significantly differ- 
ent. A\bth the stereo-camera method 25 observers 
showed a difference between test and retest of not 
more than 0.2 mm while 26 were within this tolerance 
with the NDRC Interpupillometer. It is pointed out 
that the interpupillometer technique is less cumber- 
some and better adapted to field use because it gives 
an immediate result and is not dependent upon the 
processing of film and the measuring of the photo- 
graphic record. The camera method, however, 
possesses the advantages of being relatively objective 
and of pro^’iding a permanent record of the meas- 
urement. 

13.4 VISUAL TESTS 

Criteria of visual acuity and of determination of 
possible phorias were already part of the screening 
procedure for both Services when experimental work 
was begun on this problem. On the basis of such a 
background of experience and because the selection 
cost was not too high, these same criteria for selec- 
tion were recommended. (4) Indeed, no consider- 
able amount of experimentation was done on this 
phase of the selection problem. 

Results of the 9,675 men left to be screened at Fort 
Eustis after elimination for Army General Classi- 
fication Test and MA scores, height, age, and IPD 
indicate that 3,280 were rejected because they failed 
the visual acuity test of 20/20 vision and an addi- 
tional 152 men were eliminated because they had a 
hyperphoria of more than 0.5 prism diopters or an 
exophoria or esophoria of more than 6 prism diop- 
ters. (73) 

A careful refractive examination was made of the 
men of Class 11 at Fort Monroe. (505) Refractive 
errors and phorias were determined. Such errors 
proved to be of little consequence to the group as a 
whole, although certain errors were detected in some 
of the men. The school performance for UOE for 
both fixed and aerial courses for the seven men with 
demonstrable defects but within the limits of the 
selection criteria, did not vary to any considerable 
extent from the average of the remaining 22 men of 
the same class. 

Ophthalmograph tests were made on three classes 
at Fort Monroe. (476) No statistically significant 
differences were found between UOE error and sac- 


cadic interval, fixation time, or degree of fixation 
tremor. An examination of the data for the second 
Eustis testing (485) showed no significant relation- 
ship between visual acuity and the focus used on the 
M2 Trainer. Of the 201 cases, 121 used zero diopter 
difference in focus between eyes and only 14 used 
a difference of more than 0.5 diopter. A study was 
made (493) of the relationship of visual acuity and 
scores on several tests of stereo acuity. From these 
results it is apparent that, within the visual acuity 
range of eligibility, there is little relation between 
visual and stereoscopic acuity. In many cases im- 
proved visual acuity does correspond to improved 
stereoscopic performance but the effect is of small 
magnitude. Again at Fort Monroe, using 9 men, an 
investigation of a possible relationship between 
“near” phoria measurements and fixed target con- 
sistency showed no indication that changes in phoria 
are connected with variability in fixed target read- 
ings. (506) This statement holds both for changes 
from day to day and for changes within a single day. 

In a later study (76) the relationship between 
visual acuity and stereoscopic visual acuity was ex- 
amined in order to see whether lowering ol the pres- 
ent standard ol visual acuity would result in an in- 
crease in the number ol men passing the stereoscopic 
tests, thus increasing the pool ol men eligible for 
training as stereoscopic height finder observers. One 
thousand and fifty-two soldiers were permitted to 
take the tests of stereoscopic vision at the Stereoscopic 
Testing Center at Fort Eustis even though they failed 
to meet the usual standard of visual acuity of at least 
20/20 in each eye (unaided). Analysis of the results 
showed that although the elimination of the test for 
visual acuity would produce a slight increase in the 
number of men who qualify on the stereoscopic tests, 
the burden of such testing would be greatly increased 
because a large majority of men with vision poorer 
than 20/20 fail the stereoscopic tests. However, 
screening at any level of visual acuity is better than 
no screening at all, as the performance on the stereo- 
scopic tests of the subjects with visual acuity of less 
than 20/20 standard vision is significantly poorer 
than those with standard vision. Hence the test of 
visual acuity serves as an effective and time-saving 
screening device for the stereoscopic tests. It does 
not in any sense replace the stereoscopic tests, since, 
of the men with 20/20 vision or better, only 12 per 
cent pass the stereoscopic tests. It was recommended 
that the criterion ol 20/20 vision or better be re- 


116 


SELECTION OF RANGE FINDER OPERATORS 


tained as a selection standard. 

A study of the reliability of visual acuity testing 
by a test-retest study using a floor model Projecto- 
chart with an E slide was made on 389 cases at Fort 
Eustis. (487) The results show that there is a statisti- 
cally significant but practically negligible improve- 
ment in average acuity on retest. Only 39 per cent of 
the men shifted by more than one class interval. 
Hence it was concluded that the reliability of visual 
acuity measurements was satisfactory for screening 
test purposes. 

The Massachusetts Vision Test Kit and the Bausch 
and Lomb Ortho-Rater were used to test 288 soldiers 
at Fort Eustis to determine visual acuity. (84) The 
results indicate that these tests of visual acuity are 
satisfactory substitutes for each other and that either 
instrument can be used for the rough screening pro- 
cedures which precede the careful testing of stereo- 
scopic vision for selection of range finder operators. 

Aniseikonia 

Any ocular condition which would lead to an ap- 
parent distortion of spatial relations would obviously 
affect accuracy of ranging. Such a condition is found 
in aniseikonia, in which there are size differences in 
the images of the two eyes. 

This was studied at the Princeton Laboratory at 
Fort Monroe, fndeed, the introduction of the Eik- 
onometer as a testing instrument was for this pur- 
pose. Several cases with this defect were found in 
Class 8. (469) In order to detect aniseikonic subjects 
easily a leaf room was installed in the laboratory at 
Fort Monroe. Men in Height Finder School Classes 
8-11 were tested. (499) Four degrees of aniseikonia 
from 0 to 3 are recorded. A statistical examination of 
the data showed no significant differences in per- 
formance on the height finder between degrees of 
aniseikonia. In another study of these data (508) it 
was found that the degree of defect was much smaller 
than the values obtained for an average college stu- 
dent population and is comparable to those found 
in a highly selected population of aviation cadets. 
This is true for size differences in both the horizontal 
and vertical meridian. 

For the 71 cases at Fort Monroe, none had a size 
difference in the vertical meridian greater than 1 
per cent. In the horizontal meridian there was one 
case with a size difference of the images of the two 
eyes of 2.5 per cent and there were two other cases 


with differences greater than 1 per cent. It was found 
again, when the sum of size differences with two 
meridians were added, that there was no relation- 
ship between degree of aniseikonia and the com- 
bined UOE scores for fixed and aerial targets. It is 
pointed out that the screening for visual acuities, the 
phorias, and a degree of stereoscopic acuity probably 
will catch all cases of aniseikonia serious enough to 
affect ranging accuracy. 

13.5 the SEARCH FOR A TEST OF 
STEREOSCOPIC ACUITY 

13 . 5.1 Validation of Tests of Stereoscopic 
Vision 

It became evident early in this work that a valid 
and reliable test of stereoscopic acuity was needed. 
At this time two tests were readily available. The 
Howard-Dolman peg test was a dynamic test used 
by the Air Force but with which they were dissatis- 
fied. This test could be administered only individual- 
ly and took a long time for each subject. There was 
also a single sheet of the Keystone Test which alleged- 
ly measured stereoscopic acuity. One glance at this 
card through a stereoscope immediately convinced 
a normal subject that it was far too easy as a selection 
device for as important a job as that of range finder 
operator. At Fort Monroe, in the Princeton Labora- 
tory, two tests were developed early. The M2 Trainer 
was used as a testing device. This was in the nature 
of a job miniature. Further development introduced 
known dynamic courses in this instrument. The sec- 
ond device had to do with the development of the 
Eikonometer as a testing device for this purpose. 
These developments will be outlined in detail below. 

However in the early stages of the Eikonometer 
development and until now for the M2 Trainer test, 
each subject had to be tested individually. This led 
to a very considerable testing load. Furthermore both 
tests required highly trained personnel for adequate 
administration. Hence the search continued for an 
adequate but simpler means of screening this human 
ability. It should be pointed out that both the M2 
Trainer test and an earlier form of the Eikonometer 
test were recommended in February 1942 in the first 
selection battery. (4) The revised Eikonometer test 
and the M2 Trainer test as alternate remain in the 
most recent screening recommendation in August 
1943. (73) 


RESTRICTED 


STEREOSCOPIC ACUITY TESTS 


117 


At the Harvard Psycho-Educational Clinic a study 
was made of the Howard-Dolman apparatus under 
standard conditions of various sorts. (289) It was 
found that illumination with lumiline light sources 
eliminated shadows but made a brightness discrimi- 
nation between the two posts possible. Hence judg- 
ments could be monocularly employing a brightness 
criterion which had nothing to do with stereoscopic 
vision. The apparatus was extremely unreliable 
under all of the experimental conditions employed 
and it was recommended that it be dropped from 
consideration as a screening test. If it should be em- 
ployed, illumination should be provided by a single 
200-watt bulb above the apparatus. Such illumina- 
tion does not yield any cues in the form of brightness 
changes on the movable post. 

At Tufts College a research apparatus was de- 
veloped which was called the Tufts Trainer. (557) 
This was a job miniature of the range finder and was 
developed as a research instrument for study of prob- 
lems of ranging on either stationary or moving 
targets. It had the advantage of an easily identifiable 
and adjustable zero point. It was recommended as 
a possible training and selection instrument. Inas- 
much as real depth was measured, this instrument had 
much greater sensitivity than an instrument which 
depends on decentration as was the case with the 
M2 Trainer. However, the instrument was large and 
cumbersome and it was not recommended for adop- 
tion. Nevertheless, it was adequate as a research in- 
strument and the Tufts group validated a number of 
tests of stereoscopic vision by its use. 

Several tests were used at Tufts: (1) The Bott Test 
of Stereopsis; (2) the Keystone Test (both of which 
are static tests); (3) the M2 Trainer test; and (4) 
readings on the Tufts Trainer. Data were gathered 
from 56 members of the local Naval ROTC who took 
the Bott Test of Stereopsis twice and made stationary 
and moving target runs with the Navy and Tufts 
Trainers. The results indicate that the Bott Test is 
fairly reliable and is considerably better than the 
test-retest reliability for either the M2 or Tufts 
Trainers probably because of the apparent learning 
factor for these two instruments. 

In another experiment (566) 88 men and women 
were tested and retested with the Bott, Mark 2 and 
Tufts Trainer tests. The results showed certain varia- 
tions in the two sex groups. The scores of the Bott 
Test showed the highest reliability coefficients of 0.76 
for men and 0.83 for women. A study of intercorrela- 


tions was made between the Bott Test and the M2 
Trainer and Tufts Trainer employing both fixed 
and moving targets on the two latter instruments. 
(565) It was found that the scores of the Bott test do 
not correlate with those of either of the other two 
instruments. Nor do the scores of the M2 and Tufts 
Trainers correlate with one another. However, corre- 
lations between stationary target and moving mean 
target scores made on the same instrument are quite 
high, indicating that an individual tends to main- 
tain his constant error on the same direction and of 
about the same magnitude when the target in either 
of the instruments is moving as when it is stationary. 
Intercorrelations between standard deviation scores 
on the M2 and Tufts Trainers are significantly differ- 
ent from zero. 

In still another experiment at Tufts College (568) 
a group of 60 college undergraduates, who did not 
wear glasses, took the Stereometric Section of the 
Keystone Diagnostic Series and the Bott Test of 
Stereo Acuity. It was found that the “number wrong” 
score and the “per cent stereopsis” score correlate 
highly on the Keystone Test, with a coefficient of 0.82. 
The “number wrong” Bott score correlates 0.69 with 
these Keystone scores. Analysis of the Keystone re- 
sponses lead to the important finding that the 
threshold of stereopsis is proportional to the size of 
the test objects. Smaller decentrations are correctly 
perceived when the test objects are larger. Incorrect 
responses were predominantly for the largest test 
objects, almost to the exclusion of other incorrect 
stimuli. This criticism alone goes far toward invali- 
dating this material for diagnostic test purposes. In 
another experiment intercorrelations were deter- 
mined between results of a number of short tests 
administered to 50 Naval ROTC men and 32 un- 
selected students. The tests given were the M2 
Trainer, Tufts Trainer, Bott Test of Stereo Acuity, 
Vectographic Pursuit Apparatus, and the Wulfeck 
Group Test of Stereo Acuity. (569) The last two tests 
will be discussed later. It was found again that the 
Bott Test correlates poorly with the M2 and Tufts 
Trainer (0.20 and 0.30 respectively). Scores on the 
Vectographic Pursuit Apparatus correlates at the 
level of 0.50 with the two trainers. In another report 
(567) it was determined that there were no signifi- 
cant sex differences in the performance of 56 men 
and 32 women on the Bott, M2 Trainer, and Tufts 
Trainer tests. A memorandum from the Howe Lab- 
oratory gives the results of the Howard-Dolman and 


RESTRICTED^ 



118 


SELECTION OF RANGE FINDER OPERATORS 


Verhoeff Size-Confusion Tests on the student body of 
Andover Academy. It was found that the Verhoeff 
test had much higher reliability than the Howard- 
Dolman while the correlation between the score of 
the two tests was 0.44. 

The general conclusion from all of these results 
was that static and dynamic tests of stereopsis meas- 
ure different abilities in the human subject. Further- 
more, it became evident that the ability measured 
by static tests did not correlate with ranging per- 
formance. This decision led to the abandonment of 
static tests and emphasized the development of dy- 
namic tests for the measurement of stereoscopic 
ability as selection procedure for range finder op- 
erators. The test has the advantage of administration 
to as many subjects simultaneously as there are test 
materials available. 

However, before this decision was reached, there 
had been developed the Wulfeck Group Test of 
Stereo Acuity at the Princeton Laboratory at Fort 
Monroe. This was another form of static test. It con- 
sisted of a series of vectographic pictures showing a 
simplified reticle with a small image of an aeroplane 
either in front or in back of the fiducial mark by 
known amounts. These had to be viewed through 
Polaroid glasses. It was developed because it seemed 
to have high face validity. The Wulfeck test was 
given at Fort Eustis (498) to 165 men for whom the 
M2 Trainer and Eikonometer scores were also avail- 
able and to 473 men for whom there were General 
Classification and mental ability scores available. 
The test proved to be reliable, giving a test-retest 
correlation of 0.74. The distribution of scores was fair- 
ly normal, with something of a skewing toward the 
high scores. There was a wide distribution of scores 
from 3 to 40 which was maximum. There was, how- 
ever, a negative correlation between Wulfeck scores 
and those of either of the two dynamic tests of stereo- 
scopic ability. Small positive correlations were found 
between Wulfeck scores and scores on the Mechan- 
ical Ability Test and those on the General Classifica- 
tion Test. An item analysis was also made. Another 
report (370) outlines the results of the administra- 
tion of the Wulfeck test to 291 men at Fort Eustis 
and Class 12 of the Height Finder School at Fort 
Monroe. It was found that Class 12 showed a higher 
mean and smaller standard deviation than the gen- 
eral population although the differences are not 
statistically significant. The correlation of the Wul- 
feck test with Class 12 performance was log UOE, 


aerial -|-0.25. It should be pointed out that the sign 
of these correlations is opposite to expectation and, 
taken at face value, they imply that poor stereoscopic 
acuity as measured by the test is associated with su- 
perior performance on the range finder. At the 
Harvard Psycho-Educational Clinic, 19 unselected 
subjects were tested with the Wulfeck Test and the 
Vectographic Pursuit Apparatus. The reliability of 
the Wulfeck Test was found to be 0.83 on a test-re test 
basis. The correlation between the static Wulfeck Test 
and the dynamic Vectograph test was only 0.17. (292) 

From these results it became evident that the Wuf- 
feck Group Test of Stereo Acuity was not an ade- 
quate screening device for the selection of range 
finder operators where only the very best were ade- 
quate for such training. About this time the Army 
and Navy Aviation groups requested that the test be 
given to pilot candidates to determine if this test 
would be an adequate substitute for the Howard- 
Dolman test then in use. If such a substitution could 
be made it would obviously save a very great deal of 
time in testing administrative personnel. 

A group of 235 men already accepted for pilot 
training were tested at Randolph Field with the 
Wulfeck Test, the Howard-Dolman Test and the 
Keystone Test of Stereo Acuity. (580) The last two 
tests were administered by School of Aviation Medi- 
cine personnel. Later, at Kelly Field, 310 men were 
tested who were aviation candidates but who were 
not accepted as pilot candidates. These men were 
given the Howard-Dolman and the Wulfeck Tests 
only. The Keystone Test proved to be so easy as to be 
not diagnostic at Randolph Field and hence was 
eliminated from the Kelly Field testing. A total of 
204 of the 235 men obtained an absolutely correct 
score on the Keystone Test. 

In regard to the Howard-Dolman test it was found 
that the illumination varied at the two fields, by 
tubular fluorescent lamps at Randolph Field and by 
a single electric bulb at Kelly Field. This was a retest 
for the men at Randoph Field because they must 
have already passed the Howard-Dolman Test to 
have been accepted as pilot candidates. It turned 
out that 31 cases or 13.2 per cent failed to obtain 
the passing grade on this retest. These men were 
brought back for an additional retesting and 29 of 
the 31 succeeded in getting a better score than they 
had before, and only 1 1 still failed to meet the 
Howard-Dolman cutoff score of 30 mm. This gives 
evidence of the low reliability of the Howard- 


G 


RESTRICTED 


THE WULFECK TEST OF STEREO ACUITY 


119 


Dolman as a screening test. 

An analysis of the frequencies with which the dif- 
ferent digits appear as the last digit of the Howard- 
Dolman scores indicates a predilection for round 
numbers on the part of the examiners for both the 
Randolph and Kelly Field data. 

The distribution of scores of the Wulfeck test are 
similar to those obtained at Fort Eustis with a skew- 
ing toward the good scores. The position of the 
curves on the abscissa are similar for the two sets of 
data for the selected aviation candidates and are 
significantly moved toward the better scores as com- 
pared with the Eustis data. The means are: Eustis— 
26.33; Randolph Field-32.88 and Kelly Field-32.03. 
Little relationship was found between Wulfeck and 
Howard-Dohnan scores. 

A similar testing with the \\hdfeck Test was made 
at the Philadelphia Navy Yard (583) of 165 aviation 
candidates in eight classes. Results are essentially 
similar to those obtained at Randolph and Kelly 
Fields. All of these findings are summarized in an 
additional report. (581) It had been hoped that fly- 
ing data would be forthcoming in order to validate 
the tests for the purpose of screening pilot candidates. 
This proved impossible for the men tested at Ran- 
dolph and Kelly Fields because of the difficulties of 
following the men from one field to another during 
their training. At the Philadelphia Navy Yard the 
Flight Jackets were available for study during the 
candidates’ preliminary flying training. However, 
the Navy system of scoring candidates made such a 
validation impossible and the attempt was discon- 
tinued (582, 584). 

There is no doubt that Wulfeck Test measures 
something because of the high degree of similarity 
of score distribution from the several samples and 
because of the reliability of the test itself. The three 
highly selected aviation samples possess more of this 
ability than the relatively unselected Eustis samples. 
However, validation with range finder performance 
was negative and with flying performance was lack- 
ing. The Wulfeck Test proved useful in several in- 
stances subsequently for the elimination of indi- 
viduals who had little or no stereoscopic competency. 
One suspects that it will do a better job than the 
Keystone Test. This work again emphasized the fact 
that static tests of stereoscopic acuity are inadequate 
for screening individuals who are to be faced with a 
dynamic stereoscopic job. 

The Psycho-Educational Clinic was given the task 


of developing such a dynamic stereoscopic test. The 
M2 Trainer and the Eikonometer Tests (to be dis- 
cussed later) were already in existence and were being 
used for selection. However, at this stage of develop- 
ment, both required examination of the individual 
subject for an average time of one-half hour. This was 
an enormous expenditure of examiner time and 
hence some sort of dynamic group test of stereoscopic 
acuity was desired. Using the vectographic technique, 
the Harvard Psycho-Educational Clinic developed 
successively several instruments which were subse- 
quently tested for reliability and validity. 

The Vectograph Pursuit Apparatus is an instru- 
ment in which the individual subject was asked to 
keep contact between a reticle and a target apparent- 
ly being moved in depth by cam action. Contact is 
maintained by turning a wheel which changes the 
degree of disparities of the two target images and 
hence changes the apparent depth of the target when 
seen through Polaroid spectacles which allows each 
image only to reach either eye. This is described and 
its operation discussed in detail. (301) Ten members 
of trained height finder crews of the 36th AA Brigade 
were ranked for efficiency in actual ranging by the 
senior sergeant. The scores of these subjects on the 
Vectograph Apparatus ranged from 49.3 to 42.4. The 
four highest scores account for four of the five men 
selected by the sergeant as his best observers. (284, 
290) Four men of a height finder crew of each of two 
other batteries obtained Vectograph Pursuit Appa- 
ratus scores in exactly the rank order of efficiency as 
rated by the Battery Commander. The lowest score 
of these eight men was 42.8. 

Contrasted with these, 20 untrained men were 
given the same test. The scores ranged from 47.3 to 
20.9. Only three men of this untrained group ex- 
ceeded the worst score of the men of the trained 
group, one other man getting a similar score. Trans- 
lated into UOE the mean score of the trained op- 
erators was 3.0 UOE while that of the untrained 
group was 20.0 UOE. Reliability of the instrument 
was high (0.82) as determined by the test-retest 
method on 47 high school students. 

The Vectograph Pursuit Apparatus was standard- 
ized on a highly heterogeneous group of high school 
and college students and of soldiers. By this means a 
set of decile standard scores was evolved. (284) The 
distribution curve is very heavily skewed toward the 
higher scores. It should be pointed out that it is in 
this region of higher scores that differentiation is 


t RESTRICTEin 


120 


SELECTION OF RANGE FINDER OPERATORS 


desired if one is to skim the cream in the selection 
of range finder observers. And it is just here that good 
differentiation is lacking. 

In the same paper the effects of learning with the 
Vectograph Pursuit Apparatus are discussed. Twenty- 
six subjects were given five successive trials. The 
learning factor is not great and is practically negli- 
gible for those subjects who get good scores. 

Another experiment employed 44 subjects in (284) 
a comparison of dynamic courses and judgments of 
static settings with this apparatus. Eight of the ten 
subjects who performed best on the static test Avere 
in the top 30 per cent of the dynamic operators, 
while only four of the ten best dynamic operators 
were included in the 10 best static subjects. 

The phorias of 46 high school and college students 
were also tested. The eight subjects with measurable 
phorias all did very poorly on the apparatus. (291) 
The relation of ocular dominance and Pursuit Ap- 
paratus performance was studied on 29 subjects. The 
results indicate that subjects do better if one or the 
other of the vectographs is held constant. Subjects 
tend to obtain low scores when obliged to fixate the 
stationary vectograph with their non-dominant eyes. 

There is no correlation between Vectograph Pur- 
suit Apparatus score and intelligence, and only a 
slight correlation with the Bott Test (0.35). Some- 
what larger correlation coefficients were obtained 
with Vectograph test scores on the one hand and 
Tufts Trainer (0.50) and M2 Trainer scores (0.64) 
on the other. (287) 

In a further effort to validate the Pursuit Appa- 
ratus, an experiment was carried on with a hetero- 
geneous group of 67 soldiers of the 68th AA Brigade. 
Each man took five readings on a fixed target at over 
5,000 yards. Very great variation was found between 
subjects and in the same subject from trial to trial. 
Correlations between early or widely separated trials 
was very low. These correlations were higher for 
third and fourth (0.58) and fourth and fifth (0.48) 
trials. It is concluded that the range among subjects 
in regard to stereoscopic ability is very large, running 
from little or no ability to extremely fine discrimi- 
nation. 

The members of a Height Finder Class were tested 
at Camp Davis with the Vectograph Pursuit Ap- 
paratus. (288) They were a highly selected group 
trained in the use of the ranging instrument also. 
A total of 36 men were tested. Of these, 25 obtained 
a score of better than 48 and only two had a score 


of less than 46. Two of the three men obtaining the 
worst scores were better on the Pursuit Test than 
the poorest of the trained height finder operators 
previously tested in the Boston area. 

At this point of development, this project was 
turned over to the Committee on Service Personnel- 
Selection and Training, of the NRC, who were con- 
cerned with the further development. 

The Vectograph Pursuit Apparatus was adapted 
to test 12 men simultaneously. (284) This was ac- 
complished by providing each subject with a push 
button by means of which he could record his judg- 
ments of the time the target crossed the fiducial plane 
of the reticle. 

The Vectographic Pursuit Apparatus was im- 
proved by placing the target plates in a better posi- 
tion and installing an apparatus so that integrated 
scores could be read off immediately at the end of 
each trial and by improved recording apparatus. 

A variation of this apparatus is described as the 
Dearborn-Johnston Test for Depth Perception. (285) 
In this test a target seems to be moving back and 
forth across a fixed reticle. The subject has merely 
to count and record the number of times the target 
crosses the fiducial plane. The extent and number 
of the oscillations varies for each of three courses. 
Details of construction and of operation are given. 
The distribution of scores is heavily skewed toward 
the lower scores. Experience shows that nine subjects 
can be tested in 12 minutes. The reliability of the 
test is 0.82. 

For the final validation, arrangements were made 
to admit to the Height Finder School at Camp Davis 
a certain proportion of men who did not meet the 
current standards for selection. Three classes were 
admitted who passed all other requirements except 
for stereoscopic acuity but these were selected so 
that their scores on the M2 Trainer and Eikonometer 
Tests were representative of a random sample of the 
Army population. This procedure guaranteed a suit- 
able spread of ability on those two instruments and 
probably an associated spread on the Vectograph 
Pursuit Apparatus and the Dearborn-Johnston tests 
which were being tried as supplementary stereoscopic 
devices. 

Reliabilities in this experiment for both tests were 
lower than had been found previously. By the test- 
retest method the reliability coefficient for the Vecto- 
graph Pursuit Apparatus was 0.75, slightly higher 
than that for the M2 Trainer but lower than that 


RESTRICTED 


1 


THE VECTOGRAPH PURSUIT APPARATUS 


121 


for the Projection Eikonometer. The reliability co- 
efficient for the Dearborn-Johnston Test was 0.67, 
the lowest of all four tests of stereopsis used. (295) 
The scores of the Vectograph Pursuit Test correlated 
0.32 with the Eikonometer and 0.41 with the M2 
Trainer test. The Dearborn-Johnston Test corre- 
lated 0.21 with Eikonometer, 0.16 with the M2 
Trainer, and 0.44 with the Pursuit Test. (293) Prac- 
tice effects with the Dearborn-Johnston test are dis- 
cussed. (296) 

Validation of those tests were made against the last 
22 aerial courses when true target positions were ob- 
tained by the phototheodolite records. The following 
measures were obtained: the variability scores in 
UOE; the course error score in UOE; the “hit” score 
in per cent; the percentage of “good courses” score; 
and the sum of the course error and variability 
scores in UOE. (73) 

The variability score is a measure of the variation 
in the readings taken by the height finder operator. 
For the 22 courses this score is the median spread of 
the spread of readings of the individual courses in 
UOE. The course error score in UOE is an accuracy 
score and is the median deviation in UOE from true 
range (as determined by the phototheodolite rec- 
ords) for all of the 22 courses. 

The “hit” score in per cent is obtained by dividing 
each individual course of the 22 courses into five 
parts. Medians of those parts of an individual course 
which fall within the limits of f UOE behind true 
target position and 2 UOE in front of true target 
position are considered to be “hits”. For each course, 
the “hit” score is those parts which fall within the 
specified limits as a percentage of the total number 
of parts of the course. The criterion “hit” score is 
the median of the 22 percentages obtained for the 
last 22 courses of the examination. 

The percentage of “good courses” score is obtained 
by assuming that those courses which show (a) a 
performance variability of 4 UOE or less and (b) a 
course error of 2 UOE or less are “good courses”. 
Such “good courses” are treated as a percentage of 
the total number of courses. This score emphasizes 
data falling within limits which assure a minimum of 
director instability. The “good courses” score, there- 
fore, emphasizes the nature of data which are good 
in terms of director function. 

Finally, the sum of the course error and the vari- 
ability scores in UOE considers that the median score 
for each component variable is indicative of that 


variable’s contribution to the error variance. This 
score was also developed in terms of the type of data 
which the director can use most efficiently. 

Test-retest reliability coefficients of these five per- 
formance criteria are, in rank order: sum of vari- 
ability and course error— 0.87; variability— 0.82; 
course error— 0.68; per cent of good courses— 0.67 
and “Hits”— 0.58. (73) Intercorrelations are high (7 
of the 10 over 0.80) either positive or negative as 
must be the case when some of the criteria record 
errors and others record hits. On the basis of this 
analysis of criteria only two were considered on the 
final validation— variability score with a cutoff at 
6 UOE and the sum of the variability and course 
error with a cutoff at 9 UOE. The first of these is the 
conventional measure of performance used for 
graduation at the Height Finder School. The second 
was used as a criterion because, theoretically, this 
score defines a man’s performance in terms which 
may be related to the probability of his success as 
an integral part of a successful antiaircraft unit. 
These two criteria correlate very highly with one 
another, the coefficient of correlation for these data 
being 0.97. 

The results indicate that there is a relationship 
between Vectograph-Pursuit and Dearborn-Johnston 
scores which is better than chance at the 1 per cent 
level. The Dearborn-Johnston scores are just above 
this level while those for the Pursuit test are better. 
In neither case are the validations as good as either 
the M2 Trainer or Eikonometer scores. (297, 298) 

A description of the Dearborn-Johnston Test of 
Stereoscopic Vision (297) and a description of the 
standard administration of the Vectographic Pursuit 
Test (299) have been carefully worked out. 

As a result of this validation the final recommenda- 
tion for the selection of range finder operators in 
August, 1943 dropped the Dearborn-Johnston Test 
from consideration entirely. Recommendation is 
made that the Vectograph Pursuit Test might be 
considered advantageously as an alternative to the 
Projection Eikonometer Test. But its use was not 
recommended at that time because of developmental 
and procurement problems. 

13.5.2 Xhe M2 Trainer Test 

Early in the work at Fort Monroe it was found 
that the M2 Trainer was too easy for training pur- 


[res 


RESTRICTED 


122 


SELECTION OF RANGE FINDER OPERATORS 


poses when used in the conventional way of making 
static settings. Manual control of range change was 
not feasible because such changes, introduced manu- 
ally, were not sufficiently slow or smooth and not 
sufficiently uniform from test to test. Hence the 
Princeton Laboratory devised a modification of the 
M2 Stereoscopic Trainer by introducing power con- 
trol of range change. This was accomplished by 
coupling a range drive unit to the trainer by looping 
a rubber belt on the auxiliary range knob to connect 
with a pulley on the motor shaft. (509) With this 
modification it was possible to repeat exactly the 
same course over and over again or to change the 
course by the simple expedient of changing a cam. 

With this possibility of having the M2 Trainer a 
dynamic instrument with a standardized course, it 
was early used as a selection device for testing stereo- 
scopic acuity. Here was a ready-to-hand job minia- 
ture for use for this purpose. A manual for such 
testing was subsequently written. (151) A new form 
of motor drive was described in a subsequent docu- 
ment (155) which had certain advantages over the 
earlier modification of the M2 Trainer. This enables 
the progressive change in the stereoscopic position 
of reticle pattern and target to vary at different rates; 
it presents both an approaching and withdrawing 
course and permits a selection of drive rates. Varia- 
tion from 20 UOE to 240 UOE per half minute may 
be obtained. Furthermore, this new modification 
need not be removed when the trainer is packed in 
its storage case. A further modification introducing 
tracking errors as well as stereoscopic change was 
developed at Fort Lauderdale but this modification 
had not yet been reported at the time of writing. 

The M2 Trainer as a selection test was among the 
tests on the first battery recommended by NDRC. (4) 
This test was given at successive testings at Fort 
Eustis and it was found that the mean scores on the 
second and third trials are independent of the test 
administrator of these trials. (484) Here was some 
evidence that preliminary instructors produce dif- 
ferences which carry over to the test results but these 
differences are not large. The time required for pre- 
liminary instruction depends largely on the experi- 
ence of the instructor. As a result of this experience 
the correlations between trials were found to vary 
from 0.74 to 0.89 for aerial targets and from 0.64 to 
0.75 for fixed targets. 

One disadvantage of the M2 Trainer as a test 
instrument was that it could only be given as an 
individual test, requiring a considerable expenditure 


of both subject and instructor time. It required ap- 
proximately 20 minutes for initial instruction; 5 min- 
utes up to binocular observation and 3 additional 
minutes to see stereo. 

The correlations between fixed and aerial scores 
on the four trials varied from 0.72 to 0.86. In regard to 
the reliability of the test correlations for aerial tar- 
gets for the different trials varied from 0.74 to 0.89 
and for fixed targets from 0.64 to 0.75. These correla- 
tions are satisfactorily high. 

In another experiment dealing with the compara- 
tive reliability of fixed and aerial trials with the M2 
Trainer, it was found that deviations from true score 
in any trial are independent of deviations from true 
score in any other trial. (495) In another study it was 
determined that the subjects who see stereo imme- 
diately upon looking into the M2 Trainer performed, 
as a group, considerably better than those taking a 
longer time. The difference is largely due to per- 
formers in this group. Much of this material is sum- 
marized in another place. (370) 

As the result of the subsequent testing with mem- 
bers of the height finder classes at Camp Davis, the 
reliability of the M2 Trainer on a test-retest basis 
was found to be 0.71. (73) Validation with actual 
performance scores on the height finder in the School 
at Camp Davis give the following correlations with 
M2 Trainer scores of this picked group: for variabil- 
ity score, 0.41; and for sum of variability and course 
error scores, 0.43. Both of these correlations are 
well above the 1 per cent level and hence are highly 
significant. They are, however, smaller than the co- 
efficients obtained for the Eikonometer. A systematic 
study was made of the cutoff scores in terms of in- 
structional cost. Using a standard cutoff score of 110, 
only 40 per cent of the men would be admitted to the 
school and it would be necessary to screen 256 men 
in order to obtain a class of 1 1 1 men who would meet 
the criterion. 

As a result of these data it was recommended (73) 
that the M2 Trainer test be dropped from the selec- 
tion battery. It was further recommended, however, 
that for field and other use where the Projection 
Eikonometer may not be available, men be accepted 
for training who score highest on the M2 Trainer 
test, none being accepted who score below 90. 

13.5.3 Yhe Eikonometer Test 

The Eikonometer test in one form was recom- 
mended in the first battery for the selection of range 


restricted 



THE M2 TRAINER TEST-THE EIKONOMETER 


123 


finder operators, along with the M2 Trainer test, as 
the basis for determining stereoscopic acuity. (4) In 
the final selection battery this test is recommended 
as the only basis for such selection but with a slightly 
different cutoff value. (73) In its final form, the in- 
strument consists essentially of two small projection 
lanterns, an aluminized non-depolarizing screen, 
and a system of Polaroid viewers. Two vertical line 
images are projected on the screen by the projectors. 
These images are projected through polarizers so 
that, when viewed by the subject through the Polar- 
oid viewing system, only one line is visible to each 
eye. Each pair of polarizers and analyzers, has its axes 
of polarization at right angles to the other. By means 
of gears coupling lantern slides in the projectors, the 
line images are rotated in opposite directions. The 
lines may be set manually to an extreme deviation of 
10 degrees from vertical in either direction. When 
the motor drive is started, the lines return slowly 
toward the vertical at the rate of 1 degree every 5 
seconds. The subject fuses the images binocularly 
and sees a single line tipping toward or away from 
him. When the line appears to be vertical, the sub- 
ject stops the motor drive by pressing a switch. The 
position of the lines at the time of the subject’s re- 
sponse is determined by reading a scale on the shaft 
which rotates the slides. This is known as the Stereo- 
Vertical Test with the Projection Eikonometer. After 
practice trials, the subject is given 30 test trials in 
three groups of 10 each. The raw score is converted 
into a standard score. This instrument was originally 
built for the examination of a single subject at a 
time. However, a multiple recording device has been 
developed so that it is now possible to test six subjects 
simultaneously. 

There has been a great deal of work on the de- 
velopment of the final form of this test, both in 
regard to instrumental development and to the use 
of the instruments. The original Eikonometer was 
an instrument developed to measure quantitatively 
various kinds and degrees of abnormal perception of 
special relations of the sort met with in the condition 
known as aniseikonia. A description of the instru- 
ment used in the early series of testing will be found 
in a May 1942 report. (354) The description of the 
Multiple Projection Eikonometer and detailed des- 
cription of the administration and scoring of the 
test appeared under date of December of the same 
year. (370) This account also indicates modifications 
in both the instrument and the method of admin- 
istration which resulted from experience in testing at 


Fort Eustis. 

In Class 8 at Fort Monroe, 15 men were examined 
on the Ophthalmo-Eikonometer, 20 on the Space 
Eikonometer, and 28 in the leaf room in an effort to 
determine the presence of aniseikonic effects. The 
study was reported as early as July 1941. (469) Be- 
cause the Space Eikonometer and the leaf room 
seemed to differentiate the subjects better than the 
other test, these were also given to the men in Class 
9. Six men in the group failed to pass the standards 
for graduation. Of these, the leaf room test selected 
three but also selected as defective two men who 
passed. The data of the Space Eikonometer differen- 
tiated the two groups. Case studies of the six men who 
failed to graduate are appended. 

In the screening tests for Class 11, three tests were 
given with the Space Eikonometer: axis 90 degrees; 
axis 180 degrees; and the cyclo tests. It was concluded 
that the axis 90 degrees test gave no improvement in 
prediction over the use of the combined axis 180 
degrees and the cyclo tests, and it was decided to 
weight the axis 180 degrees test tenfold in the de- 
velopment of a single score. (471) 

A detailed study was made of the reliability of 
Space Eikonometer settings on Class 1 1 for both the 
axis 180 degrees and the cyclo tests. (482) It was 
found for the axis 180 degrees test that the variability 
from day to day is rather less than would be expected 
from the mean error score of a single day. On the 
other hand, there was evidence for the cyclo test of 
such day to day variation. In another study of re- 
peated tests on Class 12, axis 180 degrees; cyclo and 
the stereo-vertical tests were given. (483) The results 
indicate that the ability measured in all three tests 
is identical. Retest at Fort Monroe gave significantly 
different mean scores from those previously obtained 
at Fort Eustis, probably because of a learning factor 
while at the Height Finder School. It was also found 
that the Space Eikonometer gave a significantly dif- 
ferent mean from tests on the Projection Eikonom- 
eter. This again may be due to learning, inasmuch 
as the tests with the Space Eikonometer were given 
before the training of these men had begun. 

An analysis was made of the results of the second 
testing at Camp Eustis in July 1942. (492) In regard 
to reliability of the measure, the test-retest reliability, 
with administrator differences eliminated, is 0.46. 
The Eikonometer Test correlates at the 5 per cent 
level with the M2 trainer test but not significantly 
with the Wulfeck Group Test of stereoscopic ability. 
In another study it was also found that there is little 


^-rRESTRlCTEBl 


124 


SELECTION OF RANGE FINDER OPERATORS 


relation between stereo acuity, as measured by 
Eikonometer Tests, and visual acuity. (493) In other 
words, within the range of acceptable visual acuity, 
there is little if any relation between these two abili- 
ties. This finding emphasizes the need for the inclu- 
sion of a test specifically to measure stereoscopic 
acuity in any battery for the selection of stereoscopic 
range finder operators. Outside the range of visual 
acuity eligibility there is some indication of poorer 
stereoscopic performance, as would be expected. 

At the end of the third Eustis testing, stereo-vertical 
scores had been obtained from 207 men making 
comparison of distributions of scores possible. It was 
found that the distributions for the two groups were 
very similar and did not vary significantly in regard 
to any measure. (494) Hence it was concluded that 
the test had been adequately standardized. It had 
already been found that administrator differences for 
these tests were of negligible magnitude. (497) In 
this same study the reliability of the stereo-vertical 
test is given as 0.52, which is higher than that for axis 
180 degrees and more than twice as high as that for 
the cyclo test. Hence it was concluded that of these 
three tests the stereo- vertical is subject to less serious 
error. 

The introduction of a motor drive in the stereo- 
vertical test proved to eliminate much of this error. 
(500) Results of the new form correlate with the 
older form 0.44. The reliability of the new form on a 
test-retest basis is 0.42. Both the old and new motor- 
driven forms of the stereo-vertical test were validated 
on the members of Class 12 at Fort Monroe. (501) 
The new form of test correlates 0.39 with log aerial 
UOE, and 0.28 with log UOE for fixed targets. 

The number of individuals eliminated during the 
Fort Eustis testing will be found in the monthly re- 
ports. (370) A summary of the test with regard to 
validation and standardization up to July 1942 is 
given elsewhere. (357) Between June 1, 1942 and 
May 31, 1943, of the 37,500 soldiers’ records examined 
only 6,242 survived the earlier tests to be eligible to 
take the stereoscopic acuity tests. Of these, only 
1,474 succeeded in passing these tests as well. 

At this point of development, control of these 
selection experiments passed to the Committee on 
Service Personnel — Selection and Training, NRC 
and subsequently to the Applied Psychology Panel 
NDRC. Working with the selected classes at the 
Height Finder School at Camp Davis, the Projection 
Eikonometer was found to have a reliability of 0.81 


on a test-retest basis. (73) In regard to validation, the 
Projection Eikonometer test correlated —0.51 with 
variability score and —0.50 with sum of variability 
and course error score for performance on the range 
finder after training. Both correlations are in the 
right direction and both are highly significant at the 
1 per cent level. At this time a change was recom- 
mended in the score for qualification of training 
from a standard score of 110 to a standard score of 
115. This would necessitate screening 236 men (who 
had passed all of the other tests) to obtain a class of 
108 men, of whom it may be expected that 100 will 
meet the graduation criterion after proper training. 
In this same report it was recommended that the 
Projection Eikonometer alone be used as the test for 
stereoscopic vision, with the M2 Trainer Test as an 
alternate in the field or in such places as an Eiko- 
nometer instrument might not be available. Follow- 
ing this recommendation, a manual was published 
for the adjustment and operation of the Projection 
Eikonometer. (146) Illustrations of the instrument 
will be found therein. 

Another study was made of the relationship be- 
tween test scores obtained on the single and Multiple 
Projection Eikonometers. (153) Small and insignifi- 
cant differences were found between the scores for two 
groups of 100 men each tested by the two methods. 
Hence the Multiple Projection Eikonometer, which 
permits simultaneous testing of six men, is recom- 
mended for use over the single Projection Eikonom- 
eter which necessitates the testing of each man singly. 
By this means a very considerable gain in testing time 
is effected and this is further strengthened by the 
results with the multiple instrument indicating that 
the position of the viewing subject does not influence 
the test score. 


13.5.4 Relation of Visual Acuity to 
Stereoscopic Acuity 

Finally, the relationship between visual acuity and 
stereoscopic acuity was reexamined to see whether 
lowering the present standard of visual acuity would 
result in a greater number of men passing the stereo- 
scopic test, thus increasing the pool of men eligible 
for training as stereoscopic range finder observers. 
(76) To determine this, 1,052 soldiers were permitted 
to take the tests of stereoscopic vision at Fort Eustis, 
even though they failed to meet the usual standard 


RESTRICTED 

— ^ ^ tr 



MISCELLANEOUS STEREO ACUITY TESTS 


125 


of visual acuity of at least 20/20 in each eye (un- 
aided). Analyses of the results show that although 
the elimination of the test for visual acuity would 
produce a slight increase in the number of men who 
qualify on the stereoscopic tests, the burden of test- 
ing would be greatly increased because a large major- 
ity of men with vision poorer than 20/20 fail the 
stereoscopic tests. Thus, the relaxation of the visual 
acuity requirement would be uneconomical in test- 
ing time. However, only 12 per cent of the men with 
visual acuity of 20/20 or better subsequently pass 
the stereoscopic tests. The test for visual acuity, 
therefore, may be considered an effective and time- 
saving screening device for the stereoscopic tests, but 
it does not in any sense replace the stereoscopic tests 
for the selection of stereoscopic range finder oper- 
ators. 


Miscellaneous 

The tests for the selection of stereoscopic range 
finder observers have been in use in the Army at the 
three Antiaircraft Replacement Centers at Fort 
Eustis, Camp Callum and Camp Hahn. In addition, a 
number of mobile testing units were activated which 
tested Army men at training centers or isolated units 
in the field in the United States. Some time later 
Navy men were tested at the Fire Control School at 
Fort Lauderdale and also many of the men of the 
USS New Jersey were given the tests as the basis for 
the assignment of men to duty on board ship. 

There were some difficulties in regard to the vali- 
dation of these tests. In the first place, the numbers 
of students in the classes at the Height Finder School 
at Fort Monroe were small and considerable time 
was required before a sufficient number of cases 
could be collected for adequate statistical treatment. 
Secondly, and this was particularly true in the early 
phases of the war with the very rapidly expanding 
Army, the mission of the School was properly and 
necessarily to turn out as many of the best trained 
stereoscopic observers as possible. This led to great 
homogeneity of the men in the classes while hetero- 
geneity and variability among the men is necessary 
for real validation of the basis of their selection. 
This factor was overcome with Classes 5, 6, and 7 
at the Height Finder School at Camp Davis, who 
were selected so that their scores on the Projection 
Eikonometer and the M2 Trainer were representa- 


tive of a random sample of Army population, al- 
though they had passed all the other selection tests. 

There was difficulty, also, with performance cri- 
teria on the range and height finders which must 
form the basis for validation of any selection test. 
The early criterion for the Height Finder School 
was a variability score in UOE. This was unsatisfac- 
tory inasmuch as it gave no adequate estimate of 
accuracy of true range. Finally, a sum of variability 
and course error score was added to the variability 
score to take account of this factor. These have been 
discussed above, as has also the difficulty with the 
five point scoring system employed by the Navy. 

Seven tests of stereoscopic acuity were adminis- 
tered to 288 soldiers at Fort Eustis. (84) This was 
done for the purpose of determining the interrela- 
tionships among the seven stereo tests, the agreement 
of these tests in selecting men with “good” and with 
“bad” stereo vision. The seven tests of stereo acuity 
were: 

1. Wulfeck Vectographic Test 

2. Bausch and Lomb Ortho-Rater 

Stereoscopic Test 

3. Keystone Stereoscopic Tests 

4. Projection Eikonometer 

5. M2 Trainer 

6. Vectograph Pursuit Test 

7. Dearborn- Johnston Test 

An examination and statistical treatment of the 
results indicate the following: (1) there is no evi- 
dence that order of test sequence significantly influ- 
enced the scores made on any of the tests; (2) the 
intercorrelations between tests range between 0.10 
and 0.53, indicating in most cases small but signifi- 
cant positive relationship; (3) the tests show low 
agreements in picking men above the 75th percentile 
and below the 25th percentile. 

The Applied Psychology Panel has reported a 
follow-up study of the efficiency of the Projection 
Eikonometer Test in predicting the performance of 
stereoscopic height finder observers. (101) In order 
to evaluate the continued operation of the Projec- 
tion Eikonometer as a selection device, the original 
validation data obtained in the previous study, with 
a population of stereoscopic height finder operators 
from Camp Davis (Classes 6 and 7), have been com- 
pared with data from operators in subsequent classes 
(11 through 18). Using these sets of data, a compari- 
son was made of the relationship of scores on the 
Projection Eikonometer to two Height Finder School 


I RESTRICTED 


126 


SELECTION OF RANGE FINDER OPERATORS 


performance criteria— the variability score and the 
combined score, which is the sum of the variability 
score and the course error score. 

Results for the two groups are practically identical. 
When a selection standard of 115 on the Projection 
Eikonometer was used as a cutoff score, 7.1 per cent 
of the 42 eikonometer passes from the former group 
failed on variability score, as compared with 7.9 
per cent of the 126 eikonometer passes from the new 
group; and 11.9 per cent from the former group failed 
on the combined score, as compared with 10.3 per 
cent from the new group. Significant critical ratios 
were found between these percentages and the per- 
centage of failures when the Projection Eikonometer 
was not used as a selection requirement for admis- 
sion to stereoscopic training. 

13.6 tests of emotional stability 

In the early days of the war, before the extended 
development of radar, it was obvious that the range 
and height finder crews must have a high degree of 
stability. If any one of the team should break in 
combat the battery would be out of effective action 
because either no information would be fed to the 
director, or this information would be inaccurate 
and incorrect. Hence some attempt at appraisal of 
emotional instability was made early in the Fort 
Monroe testing. The two-hand coordination test has 
previously been mentioned as well as pulse rate, 
blood pressure, electrocardiogram and basal metab- 
olism. None of these seemed to correlate with range 
finder performance. (360, 367) Actually, there is no 
reason to believe that they should because the rang- 
ing performance criteria were not obtained under 
emotional stress. 

The Bernreuter Personality Inventory and the 
NRG Test were given to Classes 8, 9 and 10 at Fort 
Monroe (480, 481) with no evidence of relationship 
between the scores and performance as measured by 
2 log UOE. These results are summarized elsewhere. 
(357) These men were also interviewed by a trained 
psychologist and the data obtained did not seem to 
show any relation to height finder performance. As 
a result, similar tests were dropped from the Fort 
Monroe battery. 

However, the effects of emotional tension on 
range finder performance and the assessing of emo- 
tional stability remained still a major problem. The 
group at Brown University attacked the problem of 


determining beforehand which individuals were 
likely to break down under battle conditions, so 
that they might be screened out, eliminating the 
possibility of their occupying key positions in a 
battery. 

13 . 6.1 Experiment with British Seamen 

In October 1941 an opportunity was afforded, 
through the courtesy of the British Navy, to examine 
six men of HMS Dido, all of whom had seen con- 
siderable action. Two of these men had shown “signs 
of nervous strain,” while the other four had conspicu- 
ously continued to stay at their tasks during the 
action— two of the latter being range takers. These 
men were tested for three days at Fort Monroe and 
for four days at Providence, R. I. (2) None of the 
investigators knew which men had broken during 
action until all of the results were in. A conference 
was held about a month after the testing for a dis- 
cussion of the results. In this way it was hoped to 
get a rough initial validation of certain screening 
tests which had been developed or proposed. 

These six men were given a complete refraction 
and very complete ocular functional test as well as 
tests of ocular tremor at Fort Monroe, and also 
examination for color blindness and size of ocular 
fields at Providence. There was no indication from 
the ophthalmological tests as to who might break in 
action except one man (a range taker who had shown 
conspicuous bravery) who had a considerable ocular 
tremor. 

Tests of intelligence indicated that the men were 
of average intelligence or better, with relatively little 
variation within the group. At Monroe the men were 
given electrocardiograms which were all negative; 
basal metabolism which showed one of the very good 
men to have an abnormally low rate and also no 
pathology in breathing or blood pressure. 

Electroencephalograms were given at Providence, 
taken from four cortical areas simultaneously. For 
both of the men who had broken in combat, ab- 
normal records were obtained with the men in rest- 
ing condition. In a second period, with unexpected 
light and sound, no differentiation was apparent. 
In a third period, with two minutes hyperventila- 
tion, one of the men who had broken gave slower 
brain waves from his frontal cortex. 

In a fourth period of low oxygen, one of the men 




RESTRICTED 


3 


TESTS OF EMOTIONAL STABILITY 


127 


who had broken showed abnormal breathing, as 
well as one of the good men. The other questionable 
subject showed abnormally slow brain waves and 
an abnormal pattern of breathing. The case of one 
of the excellent men was dramatic and of the great- 
est interest, inasmuch as, after two minutes of the 
reduced oxygen technique, he showed abnormally 
slow brain waves which turned into waves of 2 per 
second of large amplitude. This was followed by an 
actual convulsion of 10 seconds duration with clonic 
jaw movements and loss of consciousness. Hence the 
electroencephalographic technique not only differ- 
entiated the men in the group who had broken in 
action but also detected another individual with 
unusual and unsuspected sensitivity of the cortex 
to low oxygen. However, the widespread adoption 
of this technique seemed impossible at the time be- 
cause of the extremely small number of individuals 
competent to interpret such data at the clinical level. 

The men were also given a psychiatric examina- 
tion. This indicated no adequate differentiation 
among the men of the group. The same was true of 
the Rorschack Ink Blot Test. Both tests rated one of 
the suspected men as an extremely well-adjusted 
individual while both differentiated the other from 
the group. 

Paper and pencil tests for neurotic tendency, self- 
sufficiency, introversion, and social dominance were 
given the men without differentiation. This was also 
true for tests of mechanical comprehension and the 
perception of spatial relations. 

A number of objective tests were also given but 
these gave no real basis for determining which men 
had broken in action. Among the objective tests 
given were the two-handed coordination, manual 
steadiness, and reaction coordination tests at Brown 
University. Certain special objective tests consisting 
of some task had been developed. The scores taken 
under normal conditions were compared with per- 
formance when severe electric shock is experienced 
and/or apprehended. One of these was a stereoscopic 
pursuit apparatus which did not differentiate among 
the men. Another ineffective technique involved the 
effect of strong electric shock on the immediate 
memory for numbers. A third technique was more 
successful. This involved the reaction time for binoc- 
ular fusion with shock and also with the galvanic 
skin reflex determined. This technique does not in- 
volve learning and the reliabilities seemed good. By 
this test the two men who had broken during action 


were differentiated from the men who had shown 
conspicuous bravery during action. A more exten- 
sive report of this material will be found in another 
place. (576) 

Although the results of this testing were by no 
means conclusive, the results were encouraging 
enough to warrant continued research on the prob- 
lem. A number of studies were made by the Brown 
University group who concentrated on the problem. 
Research in this field has always been recognized as 
one of the most difficult areas in experimental psy- 
chology. The determination of emotionally prone 
individuals has always been considered to fall largely 
in the field of psychiatry. No objective psychological 
measures had been devised up to this time and hence 
the research was practically in a virgin field. 

The apparatus for determining the reaction time 
for binocular fusion with shock is described and the 
method of administration of the test is outlined. 
This is a test which must be given individually and 
which requires considerable time for each subject. 
(117) Certain easy conditions were set up in which 
the images to be fused were close together while in 
other conditions the images to each eye were far apart 
and fusion was difficult. The reaction times for the 
difficult condition proved to be reliably longer than 
those for the easy condition. Experimenting with 
laboratory subjects, it was found that there were 
noticeable individual differences with regard to the 
effect of electric shock. A few subjects show little 
effect of shock, but others show a noticeable length- 
ening in the time required for fusion when shock 
was applied. In certain of the series, apprehension 
of shock was employed. The trials occurred when 
electrodes were placed on the subject but no shock 
was given. Again individual differences were found 
in regard to the effect of apprehension. Very notice- 
able are those cases in which apprehension slowed 
down the fusion response. The detailed report of 
the experiment on the men from the HMS Dido has 
already been discussed. (118) 

Very early in 1942 a situation developed by which 
it was hoped to obtain some validation of these tests 
against battle experience. At Fort Monroe a skeleton 
battery was formed which was to be sent to some 
“hot spot” in Britain and be placed alongside a 
British antiaircraft battery in an effort to obtain an 
assessment of the systems of fire control and of the 
effectiveness of the weapons of the two armies. Op- 
portunity was given to test these men for emotional 


RESTRICTEI^ 


128 


SELECTION OF RANGE FINDER OPERATORS 


Stability. Unfortunately the skeleton battery was 
never sent overseas as a unit and hence no validation 
data was obtained from this experiment. A total of 
43 of the 48 men completed all of the tests. Two 
personality tests (NRG Personality Inventory and 
the Willoughby Test), three tests of motor perform- 
ance (stereoscopic reaction time, steadiness and pur- 
suit test), a test of immediate memory, an intelligence 
examination (Wunderlich Personnel Test), and 
measurement of change in skin resistance were given 
each subject. The effects of electric shock were noted 
in all the performance tests. Each man was tested in- 
dividually, except in the paper and pencil tests. Each 
of the tests is described and alternate methods of 
scoring are discussed. The men were also given an 
examination by a trained psychiatrist. The men 
were placed in rank order of susceptibility to emo- 
tional break under stress but, as was pointed out 
above, this group was not sent overseas and hence no 
validation from combat experience was obtained. 

Results of the testing of 207 undergraduates at 
Brown University are reported. (120) They were all 
given a group of four paper and pencil tests; the 
NRG Neurotic Inventory, the NRG Troublemaking 
Inventory, the Willoughby Test and the Wunderlich 
Personnel Test. Gontrasted with those of the stand- 
ardizing group, the results indicate relatively higher 
sides on the NRG and Willoughby neurotic inven- 
tories; their troublemaking scores do not differ ap- 
preciably from those of a standard group, and they 
give relatively high scores on the Wunderlich Per- 
sonnel Test. A further analysis of the data for the 
NRG Neurotic Inventory indicates that the differ- 
ences between the scores of the Brown students and 
the standard group may be attributed to factors of 
intelligence and/or educational opportunity. (121) 
The results are merely suggestive and are obtained 
by contrasting the scores of the Brown students with 
those of the Fort Monroe skeleton battery, some 
sailors at the New London Submarine Base, and the 
sailors from HMS Dido. 


13.6.2 Validation of Psychological Tests 

An opportunity at validation of these emotional 
stability tests was afforded by opportunity to work 
at the U. S. Submarine Base at New London. Here 
the men who have volunteered for submarine service 
are given training for this special work. Part of this 


instruction is training in the use of the “escape lung”. 
At the hnal stages of this special training the men 
must step out into a tank of water 30 feet below the 
surface and slowly rise to the surface, utilizing the 
emergency apparatus. The men can also volunteer 
to come up from a level 100 feet below the surface. 
It had been found that a certain small proportion of 
the men refused to take the escape lung tank test, 
even though they had already demonstrated to their 
own satisfaction that the escape lung was entirely 
adequate. Some had sufficient upset so that they had 
to be removed from the tank. All such cases were sent 
back to surface ships. A first report (122) outlines the 
results of testing the first 25 such individuals. This 
number was subsequently greatly increased and the 
enlarged group will be discussed as a whole below. 
However the report indicates that the reliability of 
the reaction time to fusion test, on a test-retest basis, 
is above 0.50 which is satisfactorily high. Gase his- 
tories of 26 of the submarine men who failed or re- 
fused tank escape training are given in the report. 

Although the reaction time to binocular fusion 
test seemed to validate best with emotional insta- 
bility as a predictive instrument, it must be given 
to each subject individually and requires consider- 
able time for administration. The search was con- 
tinued for some paper and pencil test that would 
correlate highly with the performance test and could 
be given simultaneously to a group of subjects. The 
early testing indicated that the NRG Neurotic In- 
ventory gave the greatest promise for this substitu- 
tion. An analysis was made of this test from the scores 
of 316 individuals (soldiers, sailors, and college 
students). (123) These subjects were fractionated into 
groups of comparable intelligence and the results 
indicate that the scores of the NRG Neurotic Inven- 
tory are indeed a function of intelligence. From the 
level of very low intelligence to average, there is no 
change in the position of the percentile curves for 
homogeneous intelligence groupings as intelligence 
increases. As intelligence increases from average to 
a higher level, the percentile curves for homogeneous 
intelligence groups are displaced more and more 
toward the high score extreme. Hence a rough cor- 
rection, based on intelligence score, is suggested. 
Also a preliminary analysis of the scores of this test 
from 886 “normals” and 37 “cases” indicates that 
a new method of scoring may be desirable in the 
application of the test to service personnel. 

The next attempt at validation reports the admin- 


RESTR 


TESTS OF EMOTIONAL STABILITY 


129 


istratioii of a battery of tests to 41 men who had 
shown emotional instability and comparison of these 
with a standard group of 72 individuals. (124) The 
emotionally unstable group were all from the New 
London base. The results indicate that the binocular 
fusion reaction time test was promising, inasmuch 
as one measure placed 66 per cent of the validating 
cases with a certain score range, while only 18 per 
cent of the standard subjects fall within the same 
range. Also, the NRG Neurotic Inventory score, 
corrected for intelligence, seemed promising on the 
selective side. About four times as many validating 
cases score above the 90th percentile as did standard 
subjects. The ^Villoughby test did not seem promis- 
ing. Finally, the setting up of arbitrary scores of a 
battery would screen out 86 per cent of the validating 
group and only 29 per cent of the standard. 

A standardized form of the final form of the appa- 
ratus for testing binocular fusion reaction time and 
rules for the administration of this test are given in 
detail. (125) 

A comparison is made of the Wunderlich and Otis 
Intelligence Test scores for submarine men and 
Brown University students. (128) These tests were 
given to 412 submarine men and 159 students, in 
order to establish norms for the conversion of the 
scores from one test to the other. This was estab- 
lished. The shock fusion results from 258 submarine 
men were compared with the psychiatric ratings of 
these same men. The reliability coefficient of the test 
was very high, and in corrected form was 0.84. The 
results indicate that a score of 0.10 constitutes a good 
cutoff value; only 26 per cent of the good men are 
rejected by this score while 60 per cent of the poor 
men are rejected. 

An attempt was made to evaluate the procedures 
used at the New London Base. (130) Comparisons of 
the total submarine school population and the sub- 
jects used in the Brown research were made in regard 
to physical examinations, escape tank performances, 
school grades and neuropsychiatric classifications. 
It was found that the population at the New London 
Base was comparable to the total population in the 
Submarine School in the light of these findings. 

The two-hand coordination test was given to a 
random selection of 257 submarine men and cutoff 
values for the test were determined. (132) An item 
analysis was carved out on results on the NRG In- 
ventory consisting of 410 “normals” and “question- 
ables” (the “good” group) and 82 “bads” and “dis- 


qualifieds” (the “bad” group). (134) The differentia- 
tion into the various groups resulted from the routine 
psychiatric examination. The item analysis resulted 
in the development of a scoring stencil for this test. 
If the 90th percentile is used as a cutoff score, the 
stencil rejects 36 per cent of the “bads” and only 4 
per cent of the “goods”. A cutoff score of 85 per- 
centile would reject 40 per cent of the “bads” and 
9 per cent of the “goods”. 

An attempt to evaluate the selectivity of these tests 
used as a battery was made upon 306 “goods” and 
71 “bads” at New London on the basis of psychiatric 
criterion. (135) Included in the battery were the fol- 
lowing tests: Otis Intelligence, Reaction Time for 
Binocular Fusion; Two-Hand Goordination Test, 
and NRG Neurotic Inventory. Passing scores were 
established for each of the tests. The NRG Trouble- 
making Inventory and the Willoughby Personality 
tests were excluded as not significant. A combination 
of three of the tests (Intelligence, Neurotic Inven- 
tory and Two-Hand Goordination), each with ap- 
propriate cutoff scores, rejected 68 per cent of the 
psychiatric “bads” and 24 per cent of the “goods”. 
It should be noted that the category of “goods” in- 
clude those classified as “questionable” in the psychi- 
atric examination. The Neurotic Inventory and the 
Fusion Reaction Time Test were found to correlate 
highly, 0.48, and hence the question was raised as to 
the advisability of retaining both tests in the battery. 
The Neurotic Inventory was retained because it was 
a group paper and pencil test, while the other re- 
quired lengthy individual examination. This study 
led to another item analysis of the NRG Neurotic 
Inventory (137) on a tested population of 728 sub- 
marine men whose psychiatric rating was known. 
Each item of the test was weighted. With a cutoff 
at the 95th percentile, the new stencil failed 3 per 
cent of the “goods” and 13 per cent of the “bads”, 
at the 90th percentile, 6 per cent “goods” and 27 
per cent “bads”; at the 85th percentile the test rejects 
9 per cent of the “goods” and 39 per cent of the 
“bads.” 

Until this time the tests at the New London Base 
had been validated only against psychiatric criteria 
which itself may be questionable. In another report 
an attempt at validation was made against tank per- 
formance and officer’s judgments as well. (140) Of 984 
men tested, 25 per cent of the psychiatric “goods” 
failed one or more tests while 62 per cent of the 
psychiatric “bads” were rejected. Of the 45 men who 



130 


SELECTION OF RANGE FINDER OPERATORS 


showed abnormal behavior in ihe tank, 30 (or 67 
per cent) failed one test or more, as against only 30 
per cent of the normal group. The results for officer’s 
judgments were less conclusive. It was believed that 
the criterion of failure on a single test was too great 
in selection cost, inasmuch as too many of the good 
men would be eliminated. Permissible rejection rate 
for psychological causes might be set at 8 to 9 per 
cent. To meet this rejection criterion the following 
schedule was set up. Failure in any two tests, or a 
score of 80 or lower on the Otis test would disqualify 
the candidate for submarine service. Applying this 
new schedule on the same population gave the fol- 
lowing results: 5 per cent of the psychiatric “goods” 
against 26 per cent of the psychiatric “bads” were 
rejected; 20 per cent of the tank failures against 7.5 
per cent of the normal group were rejected. As an 
additional criterion, blanks were sent out to the 
Fleet for officers’ judgments of performance of the 
men tested after a tour of submarine duty. The final 
battery considered in this experiment included three 
tests: Otis Intelligence and the Two-Hand Coordina- 
tion tests and the NRC Inventory. A more detailed 
analysis of the 45 escape tank failures is given else- 
where. (141) 

13.6.3 Personal Inventory Test 

The derivation, construction, administration, and 
validation of the NRC Personal Inventory are con- 
tained in another reference. (144) In its final form 
this consists of a ten page booklet with 145 items of 
forced-choice and yes-no type. These questions are 
based on a statistical analysis of 254 naval and mili- 
tary case histories, of which 140 were obtained from 
psychiatric cases and 114 from normal personnel. 
The item differentiation was determined by admin- 
istering the test to 84 early psychiatric discharges 
and 1,004 psychiatrically approved enlisted men at 
the U. S. Naval Training Station at Newport, Rhode 
Island. Sixty items differentiated between these two 
groups with a critical ratio of 2.7 or greater and a 
scoring stencil on them was devised. The validity of 
this test was determined by giving the Inventory to 
additional groups and comparing the respective per- 
centages identified. These validation results are: of 
124 Newport psychiatric cases — 52 per cent were 
identified; of 25 Chelsea Hospital psychiatric ward 
cases— 60 per cent; of 50 New London Base psychi- 


atric “bads”— 36 per cent; of 5.08 Newport Base 
normals— only 4 per cent, and of 133 New London 
Base “goods” only 2 per cent. By a split-half tech- 
nique, the reliability of the test is 0.77, which is high. 
It was concluded that the usefulness of the Inventory 
in identifying an appreciable proportion of unde- 
sirables for immediate psychiatric examination was 
demonstrated. This was important because, with the 
rapidly expanding armed forces it was impossible to 
give every new enlisted man a psychiatric examina- 
tion, as had been the previous Navy procedure, 
because of lack of trained examining personnel. De- 
tailed analysis of some of the testing at the Newport 
Naval Training Station will be found in a single 
report. (46) An extension of the analysis of the test 
battery against tank performance will be found in 
other reports. (141, 157) A summary of the escape 
tank training will be found in an appendix to this 
report. Finally, additional item analysis and evalua- 
tion of the scoring stencil for the Personal Inventory 
are reported. (167) This item analysis is based upon 
a study of the testing of 1,004 normal enlisted men 
compared with 385 early psychiatric discharges at 
the Newport Training Station. This analysis dis- 
closed that all 60 of the items included in the original 
scoring stencil continued to differentiate these two 
groups. The critical ratios of these 60 items ranged 
from 2.4 to 15.9 and 49 were common to the 60 most 
significant ones emerging from this analysis. Appli- 
cation of the stencil to a new group of 508 normal 
recruits and 184 psychiatric discharges produced a 
similar result. 

Much of this material has been gathered together 
and summarized in a single report. (156) From a con- 
sideration of these results it is recommended that 
the submarine group adopt these procedures with 
alternative plans suggested depending on allowable 
rejection rates. These tests were subsequently ap- 
plied in a number of training stations for the Navy, 
Marines and the Amphibious Forces. 

A short form of the Personal Inventory has been 
given a preliminary validation. (169) This form con- 
sists of 20 highly differentiating items from the orig- 
inal inventory, conveniently arranged on one side of 
a single sheet. It takes less than 10 minutes to give 
and can be scored at the rate of several per minute. 
Validity, as determined by comparison of scores for 
538 normal sailors and 263 psychiatric ward dis- 
charges at the U. S. Naval Training Station at New- 
port seem promising. For example, a critical score 


RESFRICTED 


THE PERSONAL INVENTORY TEST 


131 


of 8 identified 69 per cent of the psychiatric dis- 
charges and included only 41/2 per cent of the nor- 
mals. The validity of each item also proved satis- 
factory, with critical ratios ranging from 3.8 to 16.7. 
Formats A and B of the Personal Inventory Test 
were given in various ways to 2,648 enlisted men to 
determine if the two forms were comparable. (85) 
The forms are identical except in the matter of me- 
chanical arrangements for writing on the answer 
sheet. The results show that the first half of either 
test correlates with the last half of the other test as 
well as it does with its own last half. The results of 
this and other analyses indicate that Formats A and B 
are comparable. 

A short form of the Personal Inventory consisting 
of 20 highly significant items on a single sheet w^as 
developed. (89) It can be administered in 10 minutes. 
This form was used for test of 571 “boots” and 491 
special assignment men at Newport before the psy- 
chiatric interview which was given some two months 
later. Comparison of Personal Inventory scores and 
psychiatric dispositions revealed significant differ- 
entiation. For example, in the “boot” group, a criti- 
cal score of 9 on the Personal Inventory identified 
50 per cent of the discharges while including less than 
6 per cent of the non-discharges. Hence the Inven- 
tory’s usefulness in identifying an appreciable pro- 
portion of the psychiatrically undesirables seems to 
have been demonstrated. 

In an effort to evaluate the Personal Inventory 
further as a selection instrument, the service records 
were examined of 1,466 men who had taken this test 
a year before when entering “boot” training at the 
Newport Naval Training Station. (92) All of these 
men had been regarded as psychiatrically normal at 
this time testing— the discharges and psychiatric ward 
observation cases having been excluded. The object 
of this present study was to relate the following in- 
formation, whenever it was available, to the Personal 
Inventory score: (1) General Classification Test 
Score, (2) age at time of entering Navy, (3) rating 
one year after initial testing, (4) conduct record and 
(5) active or discharge status. 

The following results are true for the entire group 
and also for the 1,007 men for whom complete data 
were available. (1) The Personal Inventory identi- 
fied a significant proportion of the 52 men who were 
later discharged. Twenty-one per cent of these had 
received scores of 1 8 or above on the personal inven- 
tory as compared with but 4 per cent of the active 


men. The mean score for the discharges was signifi- 
cantly higher— 4.3 points (Critical Ratio 3.9)— than 
for the active group. Of the seven men scoring 26 or 
over on the Personal Inventory, all had been dis- 
charged. (2) The Personal Inventory showed some 
tendency to differentiate conduct cases. The mean 
score for the men with good conduct records was 
9.1 as compared with 10.8 for the men with some 
conduct offense and 1 1.9 for those with more serious 
offenses. (3) The Personal Inventory showed some 
tendency to differentiate rated from non-rated men. 
The mean score for the rated was 8.3 as compared 
with 10.0 for the non-rated men. (4) The Personal 
Inventory showed but moderate correlation with the 
old form of the General Classification Test. The 
correlation between the two was 0.28 ±0.02. This is 
sufficiently low so that the use of one test in no way 
precludes the use of the other. (5) The Personal 
Inventory showed virtually no relationship to age 
at time of entering service, the correlation being 0.01 
±0.02. (6) The General Classification Test tended 
to differentiate discharges, but to a decidedly lesser 
extent than the Personal Inventory. It tended to 
differentiate conduct cases to a slight degree, and to 
differentiate rated men somewhat more sharply than 
the Personal Inventory. It is pointed out that this 
may have been due in part to the availability of the 
General Classification Test Scores in the assignment 
of ratings. 

Another experiment from Brown University is 
concerned with the reliability of the short form of 
the Personal Inventory and its relation to the long 
form, and with the relation of scores on the long and 
short form to the General Classification Test scores. 
(94) At the Classification Center at Solomons, Mary- 
land 426 newly arrived men were given both forms 
of the Personal Inventory. The split-half reliability 
coefficient for the short form is 0.81. This coefficient 
is as high as a similar value for the long form. The 
correlation between scores on the short form and 
subsequent scores on the long form was 0.84. Conse- 
quently the short form of the Personal Inventory is 
highly correlated with the long form and its relia- 
bility is comparable as well. It was also found that 
the product-moment correlation between GCT score 
and short form score is —0.25 for the 426 men. Previ- 
ously obtained correlation between GCT and the 
long form scores was —0.35. Hence it is concluded 
that the correlation between either the long or short 
form of the Personal Inventory and the General 


restrjcTeT! 


132 


SELECTION OF RANGE FINDER OPERATORS 


Classification Test is not high enough to warrant the 
elimination of either test because of the use of the 
other. 

In a final report from Brown University is sum- 
marized an account of completed experimental re- 
search on the development of the Personal Inventory 
Test. (95) The Personal Inventory is a group test for 
the preliminary screening of psychiatrically unde- 
sirable men. Its items, based on case history dissimi- 
larities between psychiatrically undesirable and 
normal military personnel, are cast in forced-choice 
form to promote valid answering. All of the experi- 
mental results outlined in the several summaries 
above are given. The following final conclusions and 
recommendations are made. Both forms of the Per- 
sonal Inventory identify a very significant proportion 
of psychiatrically undesirable men while including 
but a small proportion of normal Naval personnel. 
Both are recommended for any or all of the following 
usages: (1) original screening; (2) selection for spe- 
cial duties where the test proves advantageous; and 
(3) equalization of units. The short form is more 
practical for most purposes. It would be expected 
that for usage, (1) men with scores above a certain 
critical score (determined on the basis of Service 
needs) would be marked for further psychiatric inter- 
view or examination; usage (2) would involve assign- 
ment of men on a basis providing favorable distribu- 
tion of Personal Inventory scores in given units; 
usage (3) would involve the Personal Inventory in 
selection for special duties. In many cases, further 
research on the relation of Personal Inventory scores 
to specific job criteria will have to be carried out. 
The long form would seem preferable for these situ- 
ations, since it would probably provide a greater 
number of significant items than the short form. It 
has been the experience of the investigators that a 
proctored group, numbering up to 500 men, may be 
given the long form Personal Inventory in an effec- 
tive manner. The largest group tested with the 
shorter form numbered about 200 men. 

A general final summary of the work of the project 


under the Applied Psychology Panel of NDRC will be 
found in a single publication for both selection and 
training of stereoscopic height finder operators. (90) 

Conclusion 

There has been reported above a very large 
amount of work having to do with the selection of 
specialized Service personnel. The net results are as 
follows: 

1. A standard set of selection tests for range and 
height finder operators was developed. Here the 
problem was to select the “cream” of the candidates 
and in this the tests were successful. The develop- 
ment of the test battery reached the point where 
there was not an undue expenditure of examiner 
time. This battery of tests was adopted early by the 
Army and somewhat later by the Navy. Too much 
emphasis cannot be placed upon the need for the 
selection of the right men for the job of stereoscopic 
operators because the success of the battery may be 
largely dependent upon their performance. This was 
especially true in the earlier days of the war, before 
the development of radar, when information for the 
director in regard to range or height was dependent 
upon the accurate and consistent operation of these 
men. 

2. There has been developed a series of tests to 
eliminate the psychologically unfit men. It is ex- 
tremely important to guard against the placing of 
men, who might break emotionally under battle 
conditions, in key jobs. The security and efficiency 
of a whole combat group might suffer as a result. 
Here the problem is essentially different from that 
of the selection of stereoscopic range finder observ- 
ers. For the latter, the very small percentage of the 
top men was desired; for the determination of the 
psychologically unfit, it was necessary to remove the 
lower or poor part of the population tested. These 
tests for the determination of the psychologically 
unfit have been used by certain groups in the Navy, 
the Marine Corps and the Amphibious Forces. 


RESTRICTED 


Chapter 14 

TRAINING OF RANGE FINDER OPERATORS 


14 1 MEASURES OF ACCURACY OF 
PERFORMANCE 

True Target Position 

O NE OF the first problems facing the Princeton 
Laboratory group at Fort Monroe was to de- 
termine the true position of a friendly aerial target in 
tri-dimensional space. Such a determination of true 
target position is essential as a basis of comparison 
of the accuracy of range-finder readings. Obviously 
these data must be more accurate than those taken 
on the same target by optical and radio range and 
direction finders. Various methods and types of in- 
struments were considered in connection with this 
problem. These included nonrecording theodolites, 
recording phototheodolites, aerial photography, 
barometric and radio altimeters, multistation micro- 
wave instruments, and others. As a result of these 
tests and for reasons of expediency and economy, the 
recording theodolites were adopted for use in all 
later tests of range finders. 

It was found that the nonrecording theodolites 
used were not accurate enough (370A, Pp 74-83) and 
that considerably greater accuracy could be obtained 
with the photo-theodolite instruments. It was also 
found that aerial photography, though extremely 
accurate, was too tedious and difficult a process for 
the purpose. A theoretical discussion of photogram- 
metry for this purpose is reported. (346) The results 
of the comparative tests of photogrammetric and 
phototheodolite methods are given in full. (362) 
The only theodolites which were available were 
the standard “spotting scopes” (PH-BC-33) used by 
the Army for photographing shell bursts in order to 
score antiaircraft target practices. Since these theodo- 
lites were not intended for precision work, it was 
necessary to modify them substantially. It was also 
necessary to develop and use an elaborate system of 
calibration techniques, computational methods and 
formulas, and auxiliary photographic and timing 
equipment in order to obtain the accuracy necessary 
for the performance tests of the optical and radio 
range finders under consideration. These are de- 
scribed theoretically (370A) and practically. (363) 
This method was finally developed to the point 


where it was used successfully, and the modified 
theodolites and accessory equipment and methods 
were adopted for regular use by the Antiaircraft Ar- 
tillery Board and the Antiaircraft Artillery Schools. 

The results of this development of apparatus and 
methods for the determination of true position of an 
aerial target are contained in a Manual of consider- 
able length. (363) Problems of communication, tim- 
ing, recording and the like are given in considerable 
detail as well as description of instruments and tables 
of organization of personnel. The method involves a 
two station system at ends of a carefully surveyed 
baseline with a synchronized phototheodolite at each 
station. Special theodolite mounts and shelters were 
provided. The target can be observed usefully only 
if both theodolites can photograph it. Under the 
conditions employed, this restricts the working 
region to a range of about 20,000 yards from each 
theodolite station, depending on visibility and the 
film used. The area directly over each station cannot 
be used because of tracking difficulties there. Com- 
plete equipment and procedure at each theodolite 
station and at the central command post is described 
in great detail. There is also a complete and detailed 
description of the methods for analysis of the theodo- 
lite data. This involves the reading and adjustment 
of the photographic film and also the procedure of 
the computing unit. Special forms were produced to 
aid in all this work. 

In an appendix is treated the problem of errors of 
alignment in phototheodolites, their measurement, 
and the correction of phototheodolite data for their 
presence. Corrections are made for refraction, curva- 
ture, and azimuth tracking. All of the test and com- 
putational forms are illustrated and alternate com- 
putational procedures are also given. The manual 
thus outlines theory and practice by this procedure 
in such a way that the method may be readily repro- 
duced by new testing or training units. 

A theoretical study (455) indicates that refraction 
makes the target seem higher than it really is because 
the effect of refraction will be to move the image 
away from the principal point of the photographic 
plate. Corrections must be made for two station data 
because of refraction, the curvature of the earth and 
difference in altitude of the two stations. (456) A 


ll^RESTRlCTED 


133 



134 


TRAINING OF RANGE FINDER OPERATORS 


Study of camera shutter timing was made. (457) The 
Filmo cameras proved to have a shorter shutter lag 
than the Cine-Kodak. (459) 

14.1.2 Magnitude of Errors with Ml 
Stereoscopic Height Finder 

With the standard of true position of an aerial 
target solved, the question of size of the errors when 
ranging with stereoscopic range finders can be an- 
sw^ered. The most complete discussion of this topic 
will be found in a Report to the Services (30) which 
is attached to a Princeton University report from 
Fort Monroe. (367) 

Three factors are involved in this assessment; the 
repeatability of measurements (precision), the cor- 
rectness of the measurements (accuracy), and the 
uniformity of an operator’s performance from day 
to day (consistency). All three of these factors are 
discussed. In these reports an attempt is made to pre- 
sent typical data, so that a fair overall picture will 
result. However, it should be noted that the data 
upon which the report is based were all taken at the 
Antiaircraft Artillery School and hence under favor- 
able conditions. Field conditions will not always be 
favorable, and observers will not always be so well 
trained. Hence, results will not always be up to the 
level of those reported. On the other hand, precau- 
tions to avoid perspective errors, which have since 
been discovered, had at that time not been taken. 
The data, therefore, are not so good as they should 
be, with equally good personnel, once these errors 
have been eliminated. 

During 1941 and 1942 the Princeton Field Lab- 
oratory at Fort Monroe collected 325,000 or more 
height finder observations, consisting of approxi- 
mately 125,000 readings on aerial targets and 200,000 
readings on fixed targets. Observations were made by 
special test observers, assigned to NDRC, and by 
student observers in the Coast Artillery School. Some 
of the test observers had had considerable training 
and experience but it is not known how they com- 
pare with seasoned observers attached to antiaircraft 
batteries. 

The tests were run for a variety of purposes, thus 
providing level and dive courses, and crossing, ap- 
proaching and receding courses at various ranges. 
The data also include certain comparative tests of 
different instruments and of instruments of differ- 
ent types. 


Precision error, which measures the variability of 
height finder errors within courses, depends to some 
extent on the nature of the course, the instrument, 
weather conditions, etc., as well as on the observer. 
Nevertheless, the precision error of most observers 
has fairly well defined limits under reasonable cir- 
cumstances. Even by the better observers, 1 UOE is 
about the smallest precision error which can be con- 
sistently attained, although precision error on partic- 
ular courses may occasionally be as low as 0.5 UOE. 
On level courses, the largest precision error of better 
observers is rarely much more than 2.5 UOE and 
even that of inexperienced observers is under 4 UOE 
most of the time. On dive courses, it is impossible to 
delimit precision error very accurately on the basis 
of available data, which is not extensive. Precision 
error increases to 6 to 10 UOE even for dives which 
are probably fairly shallow compared with actual 
tactical dives. However, none of the observers whose 
results are reported had had very much experience 
on courses of this type. 

Consistency error applies to the scatter of course 
corrections wdiile accuracy is determined by the prox- 
imity of corrections to zero. Therefore, in theory, if 
an observer is consistent, his readings can be made 
accurate by setting in a constant calibration, once 
the correction is known. This procedure is compli- 
cated by the fact that the observer’s calibrations 
depend on the instrument used and on observing 
conditions. In addition, the calibration may be sub- 
ject to change over a period of time. Thus the con- 
sistency error, estimated by periods of a few days, 
will describe the long-term scatter of course correc- 
tions only if observer-instrument calibrations are 
checked at reasonable intervals. 

It was found that consistency error involving both 
instrumental and operator errors runs slightly larger 
than precision error for experienced observers. The 
lowest limit of consistency error is about 1.5 UOE, 
under ordinary circumstances. A consistency error 
of 2.5 UOE is still good and it may be as high as 5 
UOE at times, even for experienced observers. Not 
very much is known about the consistency error of 
poor observers, except that for some men it may 
become almost indefinitely large. Due to the fact that 
student-observer data were collected at a time when 
consistency was not emphasized by the Coast Artil- 
lery School, it is difficult to say how much their 
consistency errors mean. However, it would seem 
that, with training, most observers ought to attain 


I RESTRICT]^^ 


PROBABLE MAGNITUDE OF OPERATOR ERRORS 


135 


a 5 UOE upper limit. Consistency on dive courses 
seems to be nearly as good as on the level courses 
run at the same time despite the fact that the ob- 
servers were not accustomed to dive courses. 

In this report, a further analysis is made of course 
attributes and height finder errors. In regard to the 
time rate of change of altitude or range, it is observed 
that, without “aided observing” of some kind, the 
time rate of change of the quantity to be measured 
may have considerable effect upon the errors of meas- 
urement. AVhen the height finder is set to read height, 
the stereoscopic observer is measuring what is essen- 
tially a function of the change in altitude,— thus the 
quantity being measured is stationary when the 
aerial target flies at constant altitude. \Vhen reading 
range, the observer is measuring a function of the 
change in range. Two major effects are to be expected 
when the time rate of change of the criterion is large, 
those of precision and lag. The precision effect is 
simply an increase in precision error due to increased 
difficulty of the stereoscopic task. Lag effects are in- 
duced by a constant (or nearly constant) time lag 
between the stimulus and the observer’s settings. The 
time Idqs may be positive or negative, as the observer 
lags behind or anticipates. The result of time lags 
is to introduce a bias in the errors, with the size of 
the bias depending on the time rate of change. The 
relevant rate for a height finder is the time rate of 
change, expressed in UOE per second, of the height, 
or of the range, depending on which is being read. 
It is evident that unless the range, or height, are 
constant, the time rates will rarely be constant over 
an entire aerial course. As a result, it may be expected 
that the bias will be subject to a trend for a course 
with a large time rate. 

Analyses of observer’s lags in relation to time rate 
show that the character of the results was such that 
it seems impossible, with present knowledge, to pre- 
dict, for a given course, what the lag will be. Lags 
vary in direction and magnitude for different observ- 
ers on the same course, and for the same observer 
even on similar courses, in spite of the fact that on 
many individual courses the correlations between 
the errors and the time rate were high. 

The type of contact employed by the observer may 
influence the magnitude of the errors. Two basic 
types of contact are to be distinguished, broken and 
continuous. In broken contact the observer deliber- 
ately breaks contact, makes a new setting, and then 
breaks again, yielding about one observation every 


5 seconds. In continuous contact the attempt is made 
to maintain contact at all times. In either case, the 
observer may indicate when he thinks he has the 
correct setting. Continuous contact is generally used 
when the time rate of change of the criterion is large, 
because of the difficulty in reestablishing a broken 
contact when the criterion is changing rapidly. How- 
ever, the question of preferred contact on level aerial 
courses is still open since not enough is known about 
the serial aerial function. Differences in precision 
alone are not sufficient to justify a decision, since the 
serial correlation (that is, the correlation of errors 
from observations separated in time on a single 
course) must be taken into account in considering 
the effectiveness of prediction. Basically the impor- 
tant question is whether the serial correlation for 
continuous contact is small enough that the increase 
in the number of observations results in an effective 
gain. Curves of serial correlations are given for two 
expert observers. The results indicate that the serial 
correlations vary considerabh, e\en for the same 
man reading on similar courses, and that there is 
variation by the observer. It is very probable, also, 
although the data presented provides no relevent 
evidence, that the serial correlation function varies 
with the type of course. Figures of descriptive courses 
are given for observers’ reading height or range, level 
or dive courses, and continuous and broken contact. 
Finally, the report contains data obtained on 11 
aerial courses which supply essentially the raw data 
for those who wish to calculate with actual height 
finder observations. 

A second report presents a brief analysis of the 
aerial target performance records of seven of the 
better stereoscopic observers of the class at the Anti- 
aircraft Artillery School at Camp Davis during 
December 1942 and January 1943. (238) The data 
consist of 50 to 60 aerial courses for each of the seven 
observers, most of these courses at constant height, 
with heights varying from 2,000 to 3,500 yards and 
with slant range up to about 10,000 yards. Several 
are night courses. Each course record consists of five 
sets of three visually recorded height readings, ap- 
proximately equally spaced, together with computed 
true target height for the center reading of each of 
the sets of three. Since height was nearly constant, 
this computed height was used for all three readings. 
The scoring procedure used in the Stereoscopic Ob- 
servers’ Course involves the medium error in UOE 
for each set of readings. The course error is the 


:ESTRltTTI>1 


136 


TRAINING OF RANGE FINDER OPERATORS 


median of the median errors and the variability 
score is the difference between the two extreme 
median errors. 

It was found that the precision errors for the seven 
men are not very different in magnitude from the 
precision errors obtained by the better observers in 
previous observers of the Stereoscopic Observers’ 
Course. The three best men in precision all had 
median precision errors of 1.5 UOE. In regard to 
variability errors, the three best men gave values of 
1.5, 2.25, and 2.5 UOE. Of these, only one also ap- 
pears among the three with the smallest precision 
error. One of the three most precise had a 50 per cent 
range of 5 UOE, the largest of the seven observers. 
It should be noted that the centers of the 50 per cent 
ranges are, in most cases, quite close to zero, indicat- 
ing that the personal corrections used were essen- 
tially satisfactory. The slight tendency for negative 
values is explained by the School’s preference for 
slightly short reading of —0.5 UOE. A theoretical 
discussion of the derivation of performance scores 
for both continuous and broken contact will be 
found in a Fort Monroe Princeton Laboratory study. 
(533) 

These performance reports are based upon a series 
of studies reported by the Princeton Laboratory at 
Fort Monroe. Eight such studies (391, 398) are con- 
cerned with the distribution of errors; eight others 
with the consistency of mean readings. (289, 296) 
Eleven studies analyze the homogeneity of variances 
or the means and dispersions of range finder read- 
ings. (407, 417) 

A statistical study is reported in an effort to deter- 
mine of the first observations on an aerial course 
are worse than the latter ones. (419) A class of 36 men 
at the Stereoscopic Observers’ School read on 48 
courses using broken contact. The results indicate 
that the first observation of an aerial course is subject 
to larger error, by about 40 per cent, than are the 
later readings. The first observation has an average 
error for all subjects of 3.2 UOE against an average 
for the later readings of 2.3 UOE. The next few 
readings may be subject to a slightly larger error 
than the subsequent observations, but the effect is 
practically negligible after the second reading and 
probably quite small even for the second. From the 
same source, a more detailed discussion and deter- 
mination of the serial correlation function for the 
same course is determined. (420) 

In another early study by the Princeton Labora- 


tory (423) it was found that the standard error of a 
range determination (based on 10 single readings) 
may run as high as 15 UOE. This implies that one- 
third of all determinations may be in error by 15 
UOE or more. A calculation of results shows syste- 
matic differences as great as 17 UOE are found be- 
tween instruments; between observers, as great as 
16.5 UOE; and between targets, as great as 9 UOE. 
The elimination of systematic errors due to instru- 
ment, observer, and target leaves a residual variation 
of about 6 UOE, expressed as standard error of a 
single determination. The minimum error which 
can be attributed to the observer is of the order of 
1.4 to 2.8 UOE. This figure is probably well below 
the true error attributable to the observer; no data 
are available from which an accurate estimate can be 
made, but a standard error at least as high as 4 UOE 
seems probable. Some portion of the observer’s error 
may be attributed to the method of making contact 
with the target but the effect of method of contact 
varies considerably with different observers. It seems 
probable that uncontrolled temperature effects may 
account for a portion of the errors. 

2 TRAINING METHODS AND DEVICES 

Wdien the Fire Control Division of NDRC came 
into the range finder picture, training schools were 
in operation in both Services, as well as training in 
the field. Much of this training was on the actual 
ranging instrument. The M2 Stereoscopic Trainer 
also existed and was widely used. A separate grade 
was given for graduation in theory and records and 
in actual ranging. 

The Fire Control Division set up several labora- 
tory studies in the training field. A splendid oppor- 
tunity was available to the Princeton Laboratory at 
Fort Monroe because its activities were so closely 
correlated with those of the Height Finder School 
of the Antiaircraft Command. Indeed, the students 
of this School acted as subjects for the Princeton 
Laboratory experiments. Hence there was continual 
consideration of training and of training methods 
over a period of several years at Fort Monroe. This 
activity was continued when the Army Height Finder 
School moved to Camp Davis and was extended to 
special Navy problems at the Fire Control School at 
Fort Lauderdale. These continuing activities at 
Camp Davis and Fort Lauderdale were under the 
direction of the NRC Committee on Service Per- 


( RES I RICTED^ 


SCORING METHODS 


137 


sonnel — Selection and Training, and later of the 
Applied Psychology Panel of NDRC. 

Several difficulties were apparent early in this 
development in the evaluation of training and of 
training procedures. In the first place, the instru- 
ments themselves were unreliable. The discussion 
of these sources of error will be found elsewhere in 
this report, but it may be mentioned here that they 
involve such factors as proper calibration, effects of 
temperature changes, setting of interocular settings 
and the like. 

A second problem was accurate measurement of 
true target position. This was ultimately accom- 
plished by an accurate measurement of the altitudes 
or ranges of the aerial missions by using photo- 
theodolite records which were synchronized with the 
height-finder observations. This will be discussed 
elsewhere in more detail but some indication of the 
complications and difficulties with this extremely 
accurate method may be indicated here. (363) As a 
result of a year and a half of development, involving 
a complicated system of recording, timing, inter- 
communication, and calculation procedures, this 
system of two-station synchronized phototheodolites 
placed at surveyed positions and connected through 
a control panel with one another and with the fire 
control instruments being tested was standardized. 
Each phototheodolite records photographically on 
motion picture film the synchronizing data and the 
direction of the line of sight from itself to the target 
at which it is aimed. The developed films provide 
the data from which are computed the true positions 
of the target, usually at regular intervals of time. 
Against these true positions the readings of the fire 
control instruments or the readings of the operators 
are then checked. 

The method gives results with satisfying accuracy 
but it requires a relatively large personnel both for 
operation and for computing. Also, there is a con- 
siderable lapse of time of some days before the results 
can be made known to subjects under training. This 
is because of the time required for development of 
film, reading and correction of film for tracking 
errors, and calculation of the film data. Hence much 
of the value of the phototheodolite method is lost 
because, to be really effective, the operator should 
be able to compare his readings with true range or 
height immediately and not a week or so after the 
event. 

With the more recent development of accurate 


radar, it is possible to obtain true range immediately 
with this instrument with relatively satisfactory 
accuracy and compare this at once with the readings 
from the optical instruments. This radar check 
method has been employed at Fort Lauderdale. 

In the very early days, therefore, before accuracy 
readings were available, the emphasis of the School 
was on variability of obsrvations only, rather than 
accuracy or error from true range. Hence the varia- 
bility score in UOE was adopted by the School for 
both fixed and aerial targets. Of course, accuracy 
could be determined and personal constant errors 
determined for each individual for fixed targets pro* 
viding these had been accurately surveyed and the 
true ranges known. With the development of satis- 
factory methods of determining true range, a course 
error score in UOE was devised. (73) This is an accu- 
racy score and is the median deviation in UOE from 
true range. Other possible scores are discussed in the 
same reference. Distributions of the variability UOE 
for four classes at Fort Monroe will be found tabu- 
lated. (472) For fixed targets these have median 
values ranging from 0.9 to 1.7 UOE for the different 
subjects. The limiting values range from 0.5 to 3.0 
UOE for fixed targets. The passing grade was 2.0 
UOE for the first two classes and 1.55 UOE for the 
last two classes. For aerial UOE the passing grade 
was 3.0 UOE throughout. The medians range from 
2.35 to 3.0 UOE and the limits of variability range 
from 1.6 to nearly 8.5 UOE. 

One interesting conclusion to be drawn from these 
tables is the improvement in performance with suc- 
cessive classes as a result of training and selection. 
The lowering of the passing grade for fixed targets 
is the best indication. In the case of aerial targets 
there is little change in the median value. However, 
in the first class, 50 per cent failed to reach the pass- 
ing grade; in the next two classes failure was re- 
duced to approximately 18 per cent, and, in the hnal 
class, to less than 4 per cent (only one man failed 
to meet this graduation criterion). 

An Applied Psychology Panel report deals with 
the relationship of errors in height and slant range 
readings made by stereoscopic observers. (83) Navy 
range hnder personnel perform operations which 
provide fire control data in terms of slant range. 
Army height finder operators usually procure data 
in terms of height. In order to compare the accuracy 
of these two types of data, stereoscopic observers at 
Camp Davis, North Carolina, during the last two 


RESTIHCTFDI 


138 


TRAINING OF RANGE FINDER OPERATORS 


weeks of their course, took readings on aerial targets 
both in slant range and in height. The errors in these 
readings were computed from true ranges and heights. 
A comparison was made of variability error scores 
obtained under each of the two conditions and the 
correlation between the two variability scores was 
estimated. It was found that variability scores in 
slant range is greater than those in height by approxi- 
mately 2.5 UOE. The scatter of slant range are 
greater than those in height by approximately 2.5 
UOE. The scatter of slant range scores is also greater 
than the scatter of height scores. However, the ob- 
tained correlation between the variability scores in 
height and those in slant range is 0.71 which is too 
low to permit accurate individual prediction of one 
from the other. However, the theoretical relation- 
ship between the two scores (given by the coefficient 
of correlation corrected for attenuation) is 0.93 which 
is very high. The obtained distribution of variability 
scores for slant range readings provides an estimate 
of the distribution of variability scores to be ex- 
pected from operators of Navy range finders under 
conditions similar to those existing in the present 
experiments. The data provide a basis for appreciat- 
ing the limits of satisfactory performance in making 
slant range readings as compared with similar limits 
in making height readings. The general conclusion 
reached is that the high theoretical relationship be- 
tween the two sets of scores indicates that the task of 
reading in slant range differs from that of reading 
in height primarily in difficulty and not in other 
important aspects. 

14.2.1 Training Instruments 

A number of instruments were devised in several 
laboratories, which were suggested for use as training 
instruments for stereoscopic range finder operators. 

Such an instrument was developed at Tufts Col- 
lege. (557) One great advantage of this instrument is 
that it has an easily identifiable and adjustable zero 
point. Real depth is measured rather than decen- 
tration as in the M2 Trainer. Hence the deviations 
are greater for the Tufts instrument and greater 
accuracy may be expected. For example, on the 
M2 Trainer, a deviation of 1 UOE corresponds to a 
decentration of 0.0025 mm while in the Tufts trainer 
1 UOE corresponds to about 1 1 mm of actual depth 
displacement of the target. This instrument also was 
supplied with an integrating device which provides 


scores of average error and of variability during each 
test. It was however considerably larger than seemed 
advisable and the field of view was considerably more 
restricted than that found in the actual ranging in- 
struments. Although this instrument was entirely 
adequate and extremely useful as a research instru- 
ment, it was never recommended for adoption as a 
training instrument. This instrument is fully des- 
cribed and discussed. (557) 

Another such laboratory instrument, developed 
for laboratory research purposes and satisfactory in 
that role, was developed by Brown University. (131) 
This was never recommended as a training device, 
either; as in the Tufts trainer, the Brown trainer 
presented to the subject a situation by which a stereo- 
scopically seen reticle was kept in contact with a mov- 
ing target. In the case of the Brown instrument, this 
movement is accomplished by decentration of the 
two images. An adequate scoring device was pro- 
vided. An apparatus was developed at Harvard Uni- 
versity for measuring stereoscopic and vernier acuity 
but this was never developed as a trainer. (281) 

At the Howe Laboratory of Ophthalmology a re- 
search instrument developed to study ranging on a 
simulated high speed diving aeroplane target was 
subsequently developed into a training instrument. 
(310) This was adopted by the Navy in modified form 
and requires a special building for its installation. 
Ranging is made on a target which appears on a 
semi-transparent screen on which a moving picture 
image is projected. Hence size differences as well as 
apparent depth are produced in the simulated dive. 
A true zero setting and recording were provided. 

In regard to field apparatus, as contrasted with 
laboratory apparatus, the M2 Trainer originally ex- 
isted in both Services. In its original form, errors 
which might be due to tracking or control of range 
were set in by hand by an operator. It was found that 
manual control of range change was not feasible 
because it was not sufficiently slow or smooth or 
sufficiently uniform from test to test. This latter is 
a serious objection since it is desirable to compare 
different observers as well as successive trials of the 
same observer. No such standardization could be 
expected with manual control of the instrument. 

Hence a method of power control of range change 
was developed. (371) This was accomplished by coup- 
ling a motor driven range drive unit by a rubber 
belt to the auxiliary range knob. In this way a con- 
tinuous standard range course was presented to the 
observer, with the target approaching for 1 minute 


[\estrictld3 


TRAINING INSTRUMENTS 


139 


and then receding for 1 minute. The course could 
be changed by the introduction of different cams. In 
a similar way, tracking errors, of standard form, 
could be introduced by movement of the azimuth 
and elevation knobs. 

In connection with the activities at Fort Monroe, 
the Eastman Kodak Company developed a new 
stereoscopic training instrument. (195) The descrip- 
tion of this instrument forms the first appendix to a 
report from the Fire Control Division to the Services. 
(17) Although complicated in design, the instrument 
is essentially as follows. By means of a motor driven 
cam mechanism, this instrument presents before the 
eyes of an observer stimuli closely resembling those 
which would be received if he were observing a real 
target through a real range finder, and requires him 
to react to the stimuli by operating controls similar 
to those on standard stereoscopic instruments. His 
performance at this task is automatically recorded 
as an error curve traced on a revolving drum. Adjust- 
ments are provided by means of which the instru- 
ment can be quickly changed so as to simulate range 
finders of either stereo, coincidence, or ortho-pseudo 
type. Discussion of these three types of instruments 
will be found elsewhere in this summary. 

Inasmuch as the instrument has a true zero and an 
error score may be obtained at once, it was believed 
to have great possibilities as a training device. This 
was further enhanced by the development of a mech- 
anism for obtaining an integrated error score. The 
possibility of this type of training was attractive for 
three reasons: (1) Training could be carried on in 
all types of weather and even at night, whereas train- 
ing on actual range finders is only possible when 
targets are visible, and even then true range ordi- 
narily is not known; (2) It could be carried out in 
any location and without the cooperation of aircraft, 
whereas training with the ordinary range finder re- 
quires convenient fixed targets and actual aerial tar- 
gets; (3) It would reduce the number of range finders 
required for training purposes, which at that time 
was highly desirable because of the then critical pro- 
curement situation. A complete description of the 
instrument will be found (159) to include inter- 
changeable range, elevation, and azimuth courses. 

As a result of a joint meeting of Army and Navy 
officers in June 1942, a uniform set of specifications 
was adopted for both Services (cf. 17 Appendix 3) 
and the instrument was adopted as the M4 Range 
Finder Trainer by both the Army and Navy. This 
instrument would seem to combine all of the features 


desirable in the ideal range finder trainer. 

It was found that the task presented by the stand- 
ard Mark 2 Stereoscopic Trainer is so simple that its 
use is limited to very elementary training in stereo 
ranging. In order to adapt the trainer for more ad- 
vanced training, a motor drive was constructed which 
introduces tracking errors and range movement. (91) 
Tracking errors and range movement are provided 
by means of three motor-driven cams for a bank of 
eight trainers mounted on a table. The movement is 
transmitted from the cams to the trainers by a cable 
system. Range movement is produced by using the 
left hand range knob as a pulley around which the 
cable is wrapped. Movement in elevation and train 
is produced by attaching the cables to arms which 
are attached to the horizontal and vertical target 
displacement knobs respectively. The construction of 
this system is described and illustrated in the text. 

Results with this appliance at the Naval Training 
School at Fort Lauderdale, Florida, indicate that 
when range movement and tracking errors compli- 
cate the stereo task, men reach a plateau in learning 
by 32 days, or approximately 190 runs. Previous data 
indicated that the task presented by the unmodified 
trainer (three fixed targets) was learned in an aver- 
age of 80 runs. A training schedule for use with this 
modification is included. 

A comparison of stereoscopic trainers Mark 4 and 
M6 was made by the Princeton Branch of Frankford 
Arsenal. (245) Targets favorable to precise settings 
were used. No differences in instrument performance 
were found. The targets consisted of (1) a single 
vertical line, (2) a small circle, (3) the standard 
trainer target of an airplane silhouette, (4) a Koda- 
chrome slide of a field with haystacks, and (5) a 
tower as an internal target. It was found, for four 
observers that performance with these targets was 
markedly better than that previously obtained with 
vectographs of difficult ground targets. 

As an aid in training, a model of the optics of the 
stereoscopic height finder has been developed (154) 
and has been found useful in the teaching of theory. 
Demonstrations of principles and of certain opera- 
tions of the range finder can be made with this model 
which cannot be demonstrated with the height finder 
itself. A diagram for the construction of the model 
and of the parts required will be found in this 
reference. 

Inasmuch as most stereoscopic range finder op- 
erators are also trained as trackers an instrument for 
training in azimuth and elevation tracking has been 


^ESTRICTEfi^ 


140 


TRAINING OF RANGE FINDER OPERATORS 


devised. (77) The instrument is easily assembled and 
instructions are given for its construction and opera- 
tion. Such a tracking trainer provides a way of em- 
phasizing, at an early point of the range finder 
course, the importance of good tracking as a pre- 
requisite of good readings by the range finder ob- 
server. The trainer is used to give practice in smooth 
and continuous tracking. A scoring device gives im- 
mediate knowledge of the extent of the tracking 
errors. 

Finally, some preliminary suggestions for a stereo- 
scopic spotting trainer have been advanced. (148) 
Spotting with the range finder is primarily a Navy 
requirement. The suggested trainer is a device for 
simulating an aerial target within the reticle field of 
a stereoscopic range finder. The presence of shell 
bursts is represented by white or colored lights which 
appear at various desired ranges, elevations, and azi- 
muths within the stereoscopic space. Tracking errors 
are simulated by providing that the target and shell 
bursts execute irregular vertical and lateral move- 
ments during the course of all observations. Detailed 
plans of construction are given. 

14.2.2 Yhe Training Program 

Most of the training program is to be found in the 
Service manuals and is the result of many years of 
Service experience. Much, however, has been the 
result of work at the Fort Monroe Princeton Labora- 
tory, and some has been contributed by laboratory 
studies elsewhere, although this has not been the 
primary purpose of these latter investigations ex- 
cept in the case of the studies at the Fire Control 
School at Fort Lauderdale. 

The Howe Laboratory, investigating the amount 
of practice advisable in ranging on a simulated high 
speed diving target (310) they found that at least 
180 aerial courses or their equivalent was a reason- 
ably safe minimum. This finding is the result of train- 
ing 1 1 observers on the instrument described above. 
It was found that improvement in continuous con- 
tact performance significant at the 1 per cent level 
was observed after an average of 165 courses had 
been ranged. Improvement in make-and-break con- 
tact performance was observed after an average of 
141 courses were ranged by each subject. 

At Tufts College a study was made of the effect of 
knowledge of results on training of ranging a moving 


target. (563) The method used was an automatic 
buzzer signal sounding whenever the subject’s error 
in ranging became greater than ±:5 or dz2.5 UOE. 
The subject, with this knowledge, was instructed to 
try to do so well that he would keep the buzzer from 
sounding. The results indicate that the type of 
knowledge training reduced both constant error and 
variability in ranging. These results emphasize the 
importance of immediate knowledge of results of the 
ranging during operator training and also the im- 
portance of a training instrument with a true zero. 
Both of these factors are incorporated in the Eastman 
M4 Trainer and both are lacking in the older M2 
Stereoscopic Trainer. 

Relative Effectiveness of 3 Training 
Instruments 

An intensive study of the relative effectiveness of 
the Ml Height Finder, the M2 Trainer and the East- 
man M4 Trainer in the training of stereoscopic 
height finder observers was made by the Princeton 
Laboratory at Fort Monroe. (360) The subjects con- 
sisted of 36 students in a class at the Height Finder 
School. They were chosen from a group previously 
selected at Fort Eustis by the selection tests discussed 
above. Three groups of 12 men each were famed for 
training. The first 2 weeks of the course were devoted 
to training in theory, care, and operation of the 
height finder for all three groups of subjects. The re- 
maining 10 weeks were devoted primarily to training 
in stereoscopic observations. During this period 
Group A practiced the full 10 weeks on the Ml 
Height Finder. Group B practiced several weeks on 
the modified M2 Trainer and spent the last 3 weeks 
on the Ml Height Finder. Group C spent the first 7 
weeks on the Eastman Trainer and the last 3 weeks 
on the Ml Height Finder. 

All three groups participated in three special tests 
on the height finder reading on both fixed and aerial 
targets. The first test came at the end of the fourth 
week when each man read 12 fixed target courses, 
8 aerial height courses and 7 or 8 aerial range courses. 
The second test came at the end of 7 weeks when 
each man read 10 fixed target courses and three aerial 
height courses. The final test, at the end of 10 weeks, 
required each man to read 8 fixed target and 8 aerial 
courses. All three groups received the same training 
throughout in theory, care, and operation of the 
height finder, in keeping and computing records, in 
tracking and recording on the height finder, and in 


(3 


RES FRIG FED 


\ 


THE STEREOSCOPIC OBSERVERS’ COURSE 


141 


all other aspects of training except that from the sec- 
ond through the seventh week Groups B and C made 
no stereoscopic observation with the height finder. 
All measurements were made in precision error score, 
which was currently in use by the school. 

The results indicated that men can be trained 
effectively as stereoscopic observers on either the 
modified M2 Trainer or the Eastman Trainer. Stu- 
dents require about 125 courses on either training 
instruments or the height finder, at least 25 of which 
are on the height finder, to become proficient ob- 
servers. 

The recommendations of this report were adopted 
and put into effect by the Army. Something over 6 
months’ experience utilizing the newer training 
methods, with emphasis on the use of the training 
instruments in the early stages are reported. (368) 
This report is based upon the work of four successive 
classes at the Stereoscopic Observers’ Course at Fort 
Monroe and at Camp Davis and indicates the ex- 
perience of training 134 men. 

Objectives of Course 

The course had the following objectives: (1) To 
teach the student how to make the preliminary ad- 
justments required on a height finder, such as focus- 
ing the eyepieces, setting the interpupillary distance, 
and checking the height of the image. Training in- 
struments are excellent for this purpose. (2) To give 
the student practice in using a binocular instrument. 
In this connection the training instruments proved 
excellent in overcoming the confusion resulting from 
an inability to obtain fusion of the images of the two 
eyes. (3) To give the student practice in making 
stereoscopic contact. The student was taught how to 
use the instrument, with emphasis on bracketing 
methods, the use of the fine elevation control, and 
reading when faced by a time interval device. The 
student practiced using both broken and continuous 
contact. The training instruments were adequate for 
simulating different types of courses on which the 
rate of change of height and range may be slow or 
fast. The relative advantage of using broken or con- 
tinuous contact for each type of course could, there- 
fore, be explained. (4) To give the student practice 
in computing records and in keeping a record book. 
This can be done as adequately with trainer data as 
with actual range finder data and such computations 
may also form the basis for the determination of 
calibration corrections. (5) To give the student an 


understanding of the measures of performance which 
he computes. Hence it is possible to explain the rela- 
tive importance of the various measures from the 
standpoint of data transmission, director perform- 
ance and damage to airplanes. The advantage of 
taking frequent readings can be demonstrated and 
the student can be warned of the importance of target 
contrast, the inadvisability of changing from one 
target position (below the reticles) to another 
(above the reticles) after a calibration correction has 
been established. All of this may be adequately ac- 
complished with the training instruments. 

This preliminary period of work with the trainers 
should be followed by intensive experience with the 
range finders themselves. The objectives of this sec- 
ond part of the course are as follows: (1) To instruct 
the student in the duties of all the positions associa- 
ted with the operation of the range finder, such as 
trackers and data transmitters. The student is also 
instructed in the maintenance of the instrument and 
in the necessary care in carrying, setting up, taking 
down, storing, and transporting the range finder. It 
was found that it was particularly important to have 
drills in the removal of electrical and mechanical 
connections between the parts of the instrument 
when dismounting the range finder; how often to 
clean, oil, paint, and recharge the instrument; and 
how frequently he must check the wedge and height 
adjustments. (2) To emphasize again the importance 
of the preliminary adjustments so that the instru- 
ment may be left in a condition ready for action. 
(3) To develop the student’s reading skills acquired 
during the trainer practice. With the change to the 
actual range finder, the student learns the feel of the 
range knob, the necessary clianges of body and head 
position as the instrument moves with the tracking 
of an aerial target, and the changing aspect of the 
target itself. The use of 12 and 24 power magnifica- 
tion is taught at this time. (4) To continue the 
lessons on computations and calibration corrections. 
Emphasis was again placed on the importance of a 
stable calibration correction. Since this depends 
upon care in setting the interpupillary distance, 
proper maintenance of helium content, the use of 
diaphragms and sunshades, and frequent checks of 
wedge and height adjustments, these phases of the 
work were taught at this time. (5) To instruct the 
students in the place of the height finder in the gun 
battery and in the tactics of the battery. In this con- 
nection, the problems of transmitting the data to the 


RESl'RICTF.ry^ 


142 


TRAINING OF RANGE FINDER OPERATORS 


director and orientation and synchronization were 
treated. The lesson on tactics also included such 
problems as the range within which firing is effective, 
night reading, camouflaging, sandbagging the instru- 
ment, and the protection and decontamination of 
the instrument during a gas attack. (6) Identification 
of about 90 of the commonly used friendly and hos- 
tile aircraft, and knowledge of the tactical employ- 
ment of each type and the theatres of war in which 
each may be encountered form a part of the course. 

The experience with the 134 men of the four 
classes of the Height Finder School demonstrated 
that all of these objectives may be attained in a course 
of 12 weeks duration. During the first half the stu- 
dents practiced entirely on the M2 and the Eastman 
M4 Trainers. In the second half, the students worked 
with the actual height finders. Hence the efficiency 
of a school with a given number of height finders may 
be doubled by this procedure. 

Tables and charts are given which indicate that 
the learning limits with the trainers are reached 
within the designated time limit. The number of 
courses required to reach these limits, both fixed and 
moving, is also indicated, as is the final level of per- 
formance expressed in UOE. 

There is appended to this report (368) a detailed 
program extending over the 12 weeks period with 
the work for each morning and afternoon designated 
for each day. This course allows the student to have 
regular contact with the height finder, even during 
the training period, and arranges that the lectures 
on theory and records be closely coordinated with the 
practical work. Suggestions are made in the schedule 
arrangement to relieve the monotony of the course 
as much as possible. Suggestion is also made that 
with the elimination of some of the instruction on 
the theory of the optics of the instrument and less 
emphasis on drill, this proposed 12 weeks’ course 
could possibly be reduced to 9 or 10 weeks. In regard 
to final grades, it is emphasized that accuracy of 
reading is the essential measurement of performance 
in actual height finder performance in the field and 
the recommendation was made that the final grade 
be made on an accuracy basis. A number of cautions 
are given if such a final criterion should be adopted. 
The Fire Control Division of NDRC recommended 
the adoption of these suggestions. (24) 

Training Manual 

As a result of these recommendations, a training 
manual for stereoscopic height finder observers was 


prepared under the direction of the Committee on 
Service Personnel — Selection and Training of the 
National Research Council, and carries their recom- 
mendation for adoption. With certain changes, 
largely verbal in character, this manual was adopted 
by the Antiaircraft Command. The proposed manual 
was issued in three parts. (68, 70, and 71) 

The manual contains much more material than 
the training program outlined above. One will find 
therein the incorporation of many of the suggestions 
which have grown out of the research, both at Fort 
Monroe and the several laboratories, ranging all the 
way from maintaining a helium charge on the instru- 
ment to methods of focusing the eyepieces. Much of 
this material will be found outlined elsewhere in this 
summary. A 12 weeks’ course of study is contem- 
plated and the manual is to be used as the basic text 
of such a course. Day to day references in the manual 
are indicated for study during the training period. 
The manual is fully illustrated. 

Inasmuch as this manual, in a sense, summarizes 
much of this work, a detailed description would seem 
of value. The first chapter (11 pages) outlines the 
function of an antiaircraft battery of 90 mm guns. 
The antiaircraft firing problem is described and the 
function and importance of the height finder in the 
battery is discussed. The second chapter (11 pages) 
explains very simply how angles, ranges, and heights 
are measured and indicates how this is done with 
the optical range and height finder. The third chap- 
ter (14 pages) which describes the various optical 
elements and indicates what effect each has upon a 
light ray falling upon or transmitted through it. In 
a fourth chapter (9 pages) the principle of the stereo- 
scopic range finder is outlined and the fifth chapter 
(15 pages) indicates, in simple language and with 
many illustrations, how this is accomplished in this 
particular stereoscopic instrument. Chapter 6 (30 
pages) describes the Ml Height Finder in detail, both 
with regard to its optical system and other parts. 
Here the student becomes acquainted with the techni- 
cal terminology of the instrument. The seventh chap- 
ter (4 pages) concludes the first part of the manual 
on the theory and construction of the height finder 
and briefly outlines how this instrument works. 

Part II of the manual deals with the operation of 
the height finder. Chapter 8 (14 pages) explains how 
the instrument is set up, leveled, oriented, and syn- 
chronized, and corrected for vertical parallax. Chap- 
ter 9 (12 pages) describes the preliminary adjust- 
ments such as interpupillary setting, selection of 


j ^ESTRtCTEI^ 


TRAINING MANUAL FOR STEREOSCOPIC OBSERVERS 


143 


power and filter, locus, height of image, internal 
adjustment and setting of height range lever. This 
is followed by Chapter 10 (16 pages) which is solely 
concerned with a more detailed description of the 
internal adjuster system and its operation. Both 
methods of calibration on heavenly bodies, and the 
use of the internal adjuster scale are described. Chap- 
ter 1 1 (15 pages) lists the duties of the different mem- 
bers of the height finder crew and describes the way 
the stereoscopic observer should operate the instru- 
ment when reading ranges and heights. This chapter 
treats of such topics as the importance of smooth 
tracking, placing of the target with regard to the 
reticle, broken and continuous contact methods. The 
second part closes with Chapter 12 (3 pages) in 
which are described the rules and methods for dis- 
mantling and packing the height finder. 

In the third part the important problems of cali- 
bration and record keeping are developed. Chapter 

13 (15 pages) emphasizes the need for accurate height 
finder readings and contrasts the concepts of accu- 
racy and variability. At this point the importance of 
these factors for the director is emphasized. Chapter 

14 (7 pages) has to do with the two methods of ob- 
taining a calibration correction; while Chapter 15 
(3 pages) is an introduction of record keeping of 
both daily height finder records and the maintenance 
and of the instrument. The 16th chapter (9 pages) 
outlines the arithmetic and the meaning of the sta- 
tistical terms necessary for such record keeping. 
Chapter 17 (5 pages) is an introduction to scale read- 
ing and describes the kinds of scales found on the 
Ml Height Finder. Chapter 18 (9 pages) describes 
the methods of changing height finder errors mea- 
sured in yards into units of error. Chapter 19 (13 
pages) of this part of the manual, describes record 
forms and how they should be kept. 

The fourth part of the manual is concerned with 
the maintenance of the height finder. Chapter 20 
(11 pages) contains general directions for mainte- 
nance. Chapter 21 (20 pages) gives detailed instruc- 
tions for desiccation of the instrument, for charging 
it with helium, and for subsequent testing for helium 
purity. Both the helium purity indicator and the 
Oliver methods of helium charging and checking are 
described. The 22nd Chapter (16 pages) is concerned 
with' the optical and mechanical adjustment of the 
instrument. Such items as wedge check and tracking 
telescope collimation, and the main bearing race 
backlash test and end window adjustment tests are 
considered. Chapter 23 (9 pages) outlines the as- 


sembly, operation, and maintenance of the M2 
Height Finder indicating the differences between 
this and the M 1 instrument. In Chapter 24 (3 pages) 
are given forms and detailed instructions for main- 
tenance records. 

The fifth part of the manual deals with the use of 
the height finder in the field. Chapter 25 (4 pages) 
has to do with transportation and storage. Chapter 
26 (4 pages) describes how the instrument should be 
emplaced. In Chapter 27 (5 pages) simple rules are 
given for obtaining calibration corrections in the 
field. In Chapter 28 (10 pages) are described acces- 
sories which improve height finder operation, such 
as the use of end window stops, sun shade and cover, 
and the optical gas mask M 1-1-5. Chapter 29 (3 
pages) tells how to order replacement parts for the 
height finder. “A Gun Battery in Action” is the title 
of Chapter 30 (7 pages) and here are detailed such 
matters as communications, commands in action, the 
need for speed, height finder readings and data trans- 
mission and horizontal fire at non-aerial targets. 
Chapter 31 (2 pages) gives simple rules for the care 
of the eyes of the range finder operator. Chapter 32 
(3 pages) gives in tabulated form a summary of 
height finder errors and how to avoid them. 

The sixth and final part of the manual is entitled 
“Instructor’s Section”. Chapter 33 (14 pages) is an 
outline of the methods for selection of height finder 
observers for use both in Stereoscopic Selection Cen- 
ters and by battery commanders in the field. Chap- 
ter 34 (18 pages) describes such training instruments 
as the M2 Trainer (both modihed and unmodified) 
and the M6 Trainer and M7 Trainer with sugges- 
tions for their use in a training course. Chapter 35 (7 
pages) presents a suggested schedule for a 12 weeks’ 
course for stereoscopic observers in which detailed 
outlines are given for day to day instruction during 
the first 6 weeks. Chapter 36 (7 pages) makes teaching 
recommendations. In the final chapter, 37 (3 pages) 
will be found a list of references of Service manuals 
and instructional films within this general area. 

This manual is extremely well written and is in 
exceedingly simple and clear form so that it should 
be readily understood by any soldier with sufficient 
intelligence to pass the selection tests for a stereo- 
scopic range finder observer. The many illustrations 
are excellently selected and greatly aid the under- 
standing of the text. One cannot but help be im- 
pressed with the inclusive manner in which this field 
has been covered and with the fact that so many of 
the suggestions which had grown out of the research 




RESTRICTED 


S) 


144 


TRAINING OF RANGE FINDER OPERATORS 


of the several previous years are included in the 
discussion. 

14.2.3 Characteristics of Learning Curve 

The Applied Psychology Panel has reported a fur- 
ther study of learning curves for operators of stereo- 
scopic range hnders, particularly as these apply to 
the Naval situation. (100) This is a study of the char- 
acteristics of learning during the last 8 weeks of range 
finder operation in the 16 week course of training 
for fire controlmen (0) at the Naval Training 
Schools, Fort Lauderdale, Florida. Prior to the hnal 
8 weeks of training on the range finder with moving 
targets, 8 weeks were spent on stereoscopic trainers 
and concurrent with the last 4 weeks on these trainers, 
practice was given on range finders with fixed tar- 
gets. This investigation is a practical study of the 
progress of learning under training school conditions 
and was designed to answer practical curricular prob- 
lems of how much practice may be profitably spent 
in various types of drill. 

The data for two classes are analyzed separately to 
afford comparisons. The criterion of range finder 
proficiency used is the scatter score, which is a meas- 
ure of the variability of the operator’s range errors 
determined from “true” or reference ranges obtained 
by means of radar equipment with experienced 
operators. 

The results of the study indicate that for moving 
surface targets no statistically significant improve- 
ment can be demonstrated after the first half (2 
weeks) of the practice periods now in the school 
curriculum. This holds true for both classes. For 
aerial targets, the learning curve for one class shows 
statistically significant improvement up to the final 
week; for the other class no significant improvement 
can be shown after the half-way point in the practice 
periods has been reached. In general, it may be said 
that the learning curves of performance on the range 
finder indicate that the maximum levels of profici- 
ency demonstrated were usually reached consider- 
ably before the completion of the time allotted to 
practice on both types of targets. It is suggested that, 
for aerial targets, one of the most important causes 
for the apparent absence of improvement after 2 to 3 
weeks of the 4-week period had elapsed was the rela- 
tively easy course flown by the target and another 
was the loss of interest or motivation resulting from 
knowledge that graduation standing and assignment 


had already been determined. 

It is impossible to designate an arbitrary score as 
the dividing line between satisfactory and unsatisfac- 
tory range finder performance since insufficient in- 
formation is available concerning the range errors 
the Mark 1 Computer will tolerate. It is necessary, 
therefore, to hold as a training goal the maximum 
proficiency that can be obtained and, if terminal 
plateaus on the learning curve do occur under modi- 
fied conditions of training, the resulting over-learn- 
ing would be of definite value in reducing variability 
of the optimal proficiency of the operators. 

Recommendations 

On the basis of these results and other empirical 
evidence discussed, the report recommends: (1) 
that the 4 weeks of practice with the moving surface 
targets be reduced to 2 weeks; (2) since the above 
recommendation provides 2 additional weeks, the 
present 4 weeks devoted to aerial targets be increased 
to 6 weeks; (3) the aerial courses flown by the target 
plane be made progressively more difficult through- 
out this 6 week practice period; (4) the motivational 
devices now being used be continued and that the 
possibility of using other incentives during work on 
stationary, moving surface, and aerial targets be 
seriously investigated and adopted when practical. 

A British report gives in great detail their early 
experiences in the training of stereoscopic range 
finder operators. (303) In this report are included 
learning curves of men who had had no previous 
experience in ranging and of men who had had a 
very great deal of previous experience as range takers 
but with coincidence type of instruments employed 
in the British Navy. This latter group did less well 
possibly because of greater age or possibly because of 
prejudice against the stereoscopic type of instrument. 

A general final report of the work of the project 
under the Applied Psychology Panel will be found 
in a single publication which summarizes the results 
for both training and selection of stereoscopic height 
finder operators. (90) 

As aids in the training program two simple devices 
are suggested for use in height and range finder train- 
ing establishments. The first deals with the design, 
accuracy, construction, and use of a range correction 
computer and is reported by the Applied Psychology 
Panel. (81) In training operators of range finding in- 


RESTRICTFO 


LEARNING CURVE -TRAINING AIDS-EYESTRAIN 


145 


striiments it is necessary to have some means of meas- 
uring the precision of their operation. To do this, 
one must know the true range to the target upon 
which the student operators are ranging. Assuming 
that an accurate series of ranges can be obtained from 
a nearby point, the problem of correcting for dis- 
parity in space between a number of range finding 
instruments and the point from which the range is 
known still remains. The computational labor in- 
volved in making these corrections to a number of 
instrument stations can be largely eliminated by the 
use of a computer which, given the true range and 
the azimuth angle of the target from one point, pro- 
vides immediately and simultaneously the true 
ranges for all stations lying on an approximately 
straight line. The present memorandum describes 
the construction of such an instrument and gives in- 
structions for its use. 

The design, accuracy, construction, and use of a 
range finder slide rule is reported by the Applied 
Psychology Panel. (86) The accuracy and precision of 
range readings obtained by students with stereo- 
scopic range finders at the Naval Training School 
at Fort Lauderdale, Florida, were measured in terms 
of median error and scatter score respectively. The 
computation of these scores by longhand methods is 
laborious and lengthy. For such scores to be of prac- 
tical value for training and grading purposes, the 
time and labor of their computation must be held at 
a minimum. Use of the DeYoe slide rule permits the 
computation time of these scores to be reduced by 
approximately one-third, but the procedure is still 
tedious. The range finder slide rule described in this 
report was found to be more accurate and more con- 
venient to use than either a longhand or DeYoe 
method of computing range error scores. The new 
instrument makes possible a procedure for the com- 
putation of the median error and scatter scores which 
requires approximately only one-fifth the time of the 
longhand method and one-third the time of the 
DeYoe method. Its use is recommended for adoption 
by the Services. The new slide rule is described and 
instructions for its use are given in the report. 

As an aid in the training program, the British 


report the development of two computing instru- 
ments for the easy determination of the skill of an 
optical range finder operator. (61) One instrument 
determines the summation of the errors or bias of 
the operator, the other determines the scatter of the 
instruments. The report does not describe either 
computing device in detail. 

Operators of optical instruments and of oscillo- 
scopes in both Services have in many cases felt that 
their work has an injurious effect upon visual func- 
tions. This led to an investigation of the effect of a 
16 weeks course upon visual functions at the Naval 
Training Schools at Fort Lauderdale reported by the 
applied Psychology Panel of NDRC. (96) This course 
requires a large number of hours of practice on 
stereoscopic trainers and range finders, on tracking 
telescopes and on radar oscilloscopes, in addition to 
reading for theory courses. Measures of visual acuity 
(far and near vision), vertical phoria, lateral phoria, 
stereopsis, and color vision were obtained with the 
Bausch and Lomb Ortho-Rater and an additional 
measure of stereopsis was obtained by the Stereo Ver- 
tical Test with the Multiple Projection Eikonometer. 
These measures were obtained for an entire class of 
some 75 men at the beginning and again at the end 
of the 16 weeks course. 

It was found that: (1) there was no deterioration 
of any of the above visual functions as a result of 
this practice experience; (2) there was a slight im- 
provement in visual acuity scores for the dominant 
and non-dominant eye; (3) there was an improve- 
ment in stereopsis score on both tests with the two 
different instruments; (4) There is an improvement 
in color vision score. On the basis of these findings 
it is recommended that all Service personnel who are 
required to use their eyes in operations similar to 
range finding or oscilloscope operation, be indoc- 
trinated with the knowledge that it is much more 
probable that their visual functions will improve 
than that they will deteriorate. It should be remem- 
bered that under conditions of stress or persistent 
emotional maladjustment men may complain about 
their eyes even when thorough medical examination 
shows no obvious physical basis for the complaint. 



PART IV 


NEW DEVELOPMENTS OF RANGE FINDERS 


T he preceding chapters have to do largely with ex- 
isting range finder instruments, their construc- 
tion, the control of errors which may occur in them, 
and their operation. Many of the recommendations 
have been recognized as a mere reduction of the ef- 
fects of certain errors rather than their elimination. 
The introduction of helium as a charging gas to alle- 
viate the effects of temperature stratification is an ex- 
cellent case in point. Certain fundamental studies 
were attempted to produce new instruments and 
these experiments and designs are summarized in the 
last three chapters (Chapters 15 to 17). 

Chapter 15 deals with the development of certain 
short base range finders— first for the control of lower 
caliber antiaircraft weapons and subsequently for 
use in tanks and other armored vehicles against 
usually obscure ground targets. Comparative tests 
were made for this latter problem and various types 
of range finder fields were tried. This led to the rec- 
ommendation of a coincidence type instrument be- 
cause it was immediately available. Further develop- 


ment of a stereoscopic instrument with projected 
illuminated reticle was also strongly recommended. 

Chapter 16 reports a series of systematic studies of 
reticle design which resulted in the determination of 
a set of fundamental principles underlying such de- 
sign. Two problems seem to be of importance in the 
selection of reticle pattern— avoiding false fusion and 
eliminating the effects of the height-break error. Al- 
though fore and aft marks do not prove to be of value 
for ranging consistency or precision, the retention of 
a very simplified set of such marks may give very 
valuable cues of false fusion. The lengthening of the 
fiducial lines aids in counteracting the height-break 
error. 

In Chapter 1 7 work is indicated in the design of new 
instruments, which is still being carried on, and by 
which it is hoped that many of the sources of error, 
found in existing instruments and controlled in vari- 
ous ways, may be entirely eliminated by design and 
by the introduction of newly devised optical parts 
and their mountings. 


RESTRICTED 


147 


1 






Chapter 15 

DEVELOPMENT OF SHORT- BASE RANGE FINDERS AND THEIR 
APPLICATION TO GROUND AND AERIAL TARGETS 


15 1 THE SHORT BASE RANGE FINDER 

E arly in the NDRC contact with fire control prob- 
lems, it became evident that short-base range 
finders could have a number of extremely important 
applications. For such applications, extremely great 
precision was not required. With the development of 
automatic weapons for anti-aircraft fire, for example, 
it became of importance to know when to open fire. 
Hence it was envisaged that a range finder might be 
developed which could be set to the desired range 
and firing held until the target was within this range. 
A similar application was thought of for plane to 
plane fire control. Later an application for armored 
vehicles required the determination of the exact 
range of the target. There are also many infantry ap- 
plications desirable for rifle and machine gun fire 
and for motor and rocket projectile. Hence the East- 
man Kodak Company and the Polaroid Company 
were asked to develop range finders for these applica- 
tions. This development was to provide an instru- 
ment for short and intermediate ranges. Emphasis 
was upon ease of construction and operation. A his- 
torical statement and projected uses of the instru- 
ment will be found in Fire Control Division Report 
to the Services. (6) 

The Polaroid Corporation had already developed, 
under encouragement of the Bureau of Ordnance, 
Navy Department, a simple instrument, the Mark 1 
range finding sight. This is described in a report. 
(341) It is a combination sight and stereoscopic range 
finder of unit power. It has no viewing lens system 
whatever and consists of four mirrors combined with 
a new type of collimated sight. Ranging was accom- 
plished against a projected fixed scale and this was 
deemed less desirable than an instrument of the 
Wandermark type. This instrument had a 27-inch 
base length and a calculated mean accuracy of 67 
yards at 1,000 yards. 

In response to the Section’s request, Polaroid Cor- 
poration suggested a number of ways in which exist- 
ing binoculars could be converted to range finders 
by means of simple attachments. One of these, 
selected on the basis of ease of manufacture and use, 
was developed by the company. This is described 


in a report. (342) It uses the principle of superim- 
posed coincidence; that is, it produces within the 
same field two images of the target overlying one 
another but displaced by an amount proportional to 
the parallax of the target as seen from the two ends 
of the attachment. By rotating the ranging wedges, 
these two images move laterally and are accurately 
superimposed for that position of the wedges which 
corresponds to target distance. Although this instru- 
ment had a base length of only 6 inches and a theo- 
retical computed performance of 38 yards at 1,000 
yards when 8-power magnification was supplied, it 
was not further developed. Instead the Polaroid Cor- 
poration developed a 43-inch stereoscopic instru- 
ment with a projected Wandermark type of reticle. 

Meanwhile the Eastman Kodak Company also pro- 
duced an instrument of the superimposed coin- 
cidence type, which is completely described. (196) 
Basically this instrument is of conventional design, 
but two important modifications were introduced. 
One is the use of complementary color filters in the 
two light paths, so that red and green images of the 
target are produced. These superimpose and fuse 
into an image of normal color for that position of 
the ranging mechanism (in this case, a moveable 
mirror) which corresponds to the target distance. 
The other modification is the introduction of a 
novel but simple method by which the instrument is 
rendered auto-collomating. The introduction of such 
a device is necessary because the optics in range 
finders are subject to minor derangements due to 
temperature changes and other causes which, if un- 
corrected, would cause errors in range readings. 
Ordinarily, such instruments are provided with a 
range corrector or internal adjuster, by means of 
which the infinity setting of the scale may be checked 
and adjusted. When an instrument is so constructed 
that the reading of the range scale is not affected by 
these minor derangements, and no internal adjuster 
is required, it is said to be “auto-collimating”. In the 
Eastman instrument it was necessary only to place 
a line in the gate between two diamonds to make this 
correction. This instrument has a base length of 15 
inches, is supplied with 6-power and has a calculated 
accuracy of 20 yards at 1,000 yards. 



149 


150 


DEVELOPMENT OF SHORT-BASE RANGE FINDERS 


15.1.1 Tests of Instruments and Operators 

Inasmuch as the usefulness of instruments of this 
type would be much reduced if it was necessary to 
select as operators men with special abilities and 
then to give them a considerable training, a field 
test with unselected Army personnel was made. This 
is reported in a Report to the Services issued by the 
Field Control Division of NDRC. (23) The tests were 
made at the side of B Battery 602nd CA (AA) in the 
vicinity of New York. It was hoped that two ques- 
tions might be answered: (1) can ordinary soldiers 
use instruments of this type and (2) what accuracy 
may be expected from their use. 

In this study a total of 5,602 ranges on 8 targets 
at ranges between 209 and 1,813 yards were made by 
10 trained stereoscopic observers and by 13 untrained 
and unselected men picked at haphazard from the 
Battery personnel. None of this second group had 
previously looked through a range finder of any sort. 
Some of the targets favored the coincidence type of 
instrument, e.g., a flagpole at 1,388 yards. Others 
favored the stereoscopic type of instrument, e.g., a 
concrete abutment in shadow under a bridge at 618 
yards. Some targets had a sky background while 
others did not. Both instruments were held by hand 
throughout the tests. 

An analysis of the results indicated that, using 10 
per cent accuracy as a criterion, six of the eight 
trained observers were able to use the 15-inch East- 
man instrument satisfactorily to at least 600 yards, 
whereas six of the eight obtained satisfactory read- 
ings to at least 1,800 yards with the Polaroid instru- 
ment. It will be remembered that these observers had 
been previously selected and trained in stereoscopic 
range finding. The other two observers, though sim- 
ilarly selected and trained in stereoscopic, could not 
obtain satisfactory ranges by this criterion beyond 
200 yards with the Polaroid 43-inch and one could 
not use it at all. On the Eastman instrument, the two 
men who failed to meet the criterion at 600 yards met 
it at 400. 

Among the untrained group, seven of the thirteen 
men used the Eastman instrument satisfactorily to at 
least 600 yards, while hve of the thirteen used the 
Polaroid instrument satisfactorily to at least 600 
yards. Six of these men could not use the Polaroid be- 
yond the same range. In other words, only half of the 
untrained group could use either instrument and the 
performance of those who could was about the same 


on both instruments. In the hands of the untrained 
groups the Eastman 15-inch instrument gave better 
results on distant targets (beyond 1,000 yards) than 
the 43-inch Polaroid instrument. For the trained 
group this situation was reversed — but against it 
must be remembered that these men had had previ- 
ous stereoscopic training, particularly on targets at 
long range. 

It was discovered that bad ground background 
reduces the accuracy of both instruments in a fairly 
predictable way as compared with ranges against tar- 
gets with a sky background. Also, after the first half 
day’s indoctrination, the untrained group showed 
no appreciable increase in accuracy with either in- 
strument, though over 150 additional ranges were 
taken. However it was observed that there was a 
marked increase in the speed of making range obser- 
vations during this additional period of practice. At- 
the close of the test, the observers were asked to 
estimate the range to each of the targets without the 
aid of instruments. 

As a result of this field test, the following con- 
clusions were drawn in regard to the Eastman 15-inch 
and the Polaroid 43-inch range finders: 

1. Either instrument, even in the hands of un- 
trained observers, gives results which are consider- 
ably better than unaided range estimates. 

2. Neither instrument can be handed out indis- 
criminately like a mess kit. Some sort of selection and 
training of personnel is required. 

3. Selection of observers could consist merely of 
trying out on the instrument itself at least twice the 
number of men required for observers and replace- 
ments. 

4. The selected personnel should be given three or 
four days’ training in which, in the earlier stages, em- 
phasis should be on increasing accuracy and, in the 
later stages, on increasing the speed of range settings. 

5. If this procedure is followed, either instrument 
can be expected to have a useful range of 600 - 800 
yards. 

6. By careful selection and training, the useful 
range of the Polaroid instrument can be extended to 
at least 2,000 yards. What could be accomplished by 
careful selection and training with the Eastman in- 
strument is unknown. It is anticipated, however, that 
the 8-power 30-inch Eastman instrument now under 
construction will have a useful range of at least 1500 
yards. 

Similar examination and tests were made by the 


[kESlRlCTED 


EASTMAN AND POLAROID INSTRUMENTS 


151 


British Admiralty Research Laboratory for both the 
Eastman (48) and Polaroid (49) instruments with 
approximately similar results. As a result of these 
field tests the Polaroid Corporation made certain 
modifications in their instrument but these were 
never reported and eventually the project was 
dropped. 

Meanwhile the Eastman Kodak Company devel- 
oped this same idea into a 30-inch range finder which 
was accepted by the Army as MIO for use with the 
M5A2 Director. Description of the 15-inch instru- 
ment and instructions for its use are contained in a 
report (208) which also gives schematics of the 
optical system employed. 

The Eastman Kodak Compay has also reported a 
series of tests made with the 15-inch instrument. (199) 
The ranges employed varied from 102 to 2,058 yards. 
The data include 217 groups of 5 readings each. The 
average spread (the difference between the highest 
and lowest reading) for these data was 4.4 UOE. 
Some targets appeared to be slightly better than 
others from the point of view of precision. A high 
contrast target at 102 yards gave an average spread 
of only 2.6 UOE while a chimney at 457 yards gave 
an average spread of 5.4 UOE. In regard to accuracy 
it was found that most of the readings fell within 
the limits of plus or minus 5 UOE. However, if five 
ranges are taken and the results averaged, the average 
value will vary from true range by more than 5 UOE 
only in two or three per cent of the cases for fixed 
targets. Suggestions for improvement of the instru- 
ment are outlined on the basis of this experience. 

The Eastman Kodak Company has reported the 
description and operating instructions of two 1- 
meter range finders developed for infantry use and 
designated as T-25 and T-26. (206) The T-25 is a 
full-field superimposed-image type having two nearly 
identical images of different colors. The T-26 is of 
the split-field invert-coincidence type. Both instru- 
ments have an interior semi-auto-collimating range 
corrector system which may be set at any time by 
observing the interval range scale. The two instru- 
ments are identical in optical and mechanical de- 
tails except for the center coincidence prism assem- 
bly and its mounting frame. By interchanging eye- 
pieces, the power of the instruments may be changed 
from 11. 5x to 15x. Calculated acuity is 2.5 UOE or 
30 seconds of arc at the eye. At 1,000 yards this should 
give an average error of 1 1.6 yards with 11.5 magni- 
fication, and of 8.9 yards with 15 magnification. The 


instruments have a field angle of 4 degrees. The range 
scale is read by a second observer. These instruments 
were tested at Fort Benning by the Infantry Board. 

A 1 -meter ortho-pseudo type range finder was de- 
signed by the Eastman Kodak Company. (211) A 
description and schematic drawings are reported. 
The computations indicated that it would have a 
15x magnification and a 3-degree field and would be 
useful out to 3,000 yards. No layout drawings were 
ever made and the project was dropped. 

There is a British report of trials with a fixed scale. 
1 1/4 -meter base Levallois Stereoscopic Range Finder 
for antiaircraft use. (52) From a light AA Battery, 
117 men were given one day’s training in the use of 
the instrument and a check was taken at the end of 
the day of their ability to take ranges on an aircraft 
flying on a straight course from 5,000 meters through 
a crossing point at approximately 1,000 meters at a 
height of approximately 2,000 feet and a speed of 
150-200 mph. Of this practically unselected group, 
15 per cent produced excellent results (less than 200 
seconds of arc error), and an additional 18 per cent 
produced reasonably good results (200-400 seconds 
of arc error). Firing trials using the Levallois instru- 
ment, in conjunction with a predicator-controlled 
Bofors gun, demonstrated that with range finder co- 
operation the percentages of hits in the total number 
of rounds fired and in the line of sight rounds, are 
both markedly higher than the averages obtained 
when employing the normal drill (13.6 per cent or 
24 per cent hits in line of sight rounds as against 
7 per cent average of 2 months of practice camp 
firing). 

15.1.2 Application to Armored Vehicle Use 

It became evident during the progress of the war 
that the usual artillery methods of range estimation 
and a bracketing procedure were too slow and used 
too much of the limited available supply of ammuni- 
tion to be completely acceptable for use by armored 
vehicles. A British report tells of a firing trial using 
the Barr &: Stroud No. 12 (80 cm base, magnification 
14) against range estimation without the use of in- 
struments. (54) A bracketing procedure was em- 
ployed in both cases, but, for visual range estima- 
tion, the opening bracket was one complete turn of 
the handwheel (25 mils) while only 1/4 turn was 
used in the range finder experiments. A different tar- 


RESTRICIED 


152 


DEVELOPMENT OF SHORT-BASE RANGE FINDERS 


get was engaged each day — the ranges varying from 
815 to 2,723 yards. In all 20 tank crews were tested. 

The use of the range finder combined with the 
type of fire control described resulted in a saving of 
ammunition. It was estimated from the results of 
the trial that the use of the range finder saved li/^ 
rounds of ammunition per engagement. This con- 
clusion is based on firing at ranges up to about 
2,000 yards with two longer ranges excluded. It 
was also found that the use of the range finder 
will increase the chance of a hit with the first round 
by about 100 per cent. This conclusion is based 
on the results of the firing at all the ranges em- 
ployed. The range finder was mounted on a bracket 
on top of Sherman (M4A4) Tanks and it was 
found that the adjustment of the instrument was 
unaffected by gunshock by fire of 75 mm guns. The 
range finders were operated by the tank commanders, 
who were trained in its use for 2 hours per day for 
7 days. This amount of training gave a completely 
satisfactory standard of operation. Hence this report 
recommends the range finder as a useful fire control 
instrument for tank gunnery and advocates that it 
should be issued to tank units. 

An analysis of the actual numbers emphasizes the 
validity of these conclusions. In one set of trials with 
eleven targets varying in range from 955 to 3,785 
yards, the average error for visual estimation was 575 
yards, against the average error with the range finder 
of only 75 yards. In the case of no single target is the 
average of visual estimation better than the error 
with the instrument. In the case of only two long 
range targets is the average instrumental error more 
than 100 yards and even this is more than five times 
better than the average for estimation for these same 
two targets. In another set of trials the averages for 
six targets (from 815 to 2,723 yards) gave average 
errors for visual estimation of 374 yards and for the 
range finder of 67 yards. For still another set of trials 
for 14 targets ranging from 428 yards to 5,300 yards, 
the average errors for visual estimation are 598 yards 
and for range finder 168 yards. In these last results, 
if the two extreme targets, which are probably be- 
yond the usable range of the instrument, are elim- 
inated (4,810 and 5,300 yards) the average errors are 
428 yards for visual estimation and 87 yards for the 
range hnder. A statistical analysis of these results 
indicates that the average error of visual estimation 
of range is approximately a linear function of range 
and is approximately 30 per cent of the range at all 


ranges from 800 to 4,000 yards, and that the average 
error of range finder estimation is a square law 
function of range and is approximately 20r- (where 
r is the range in thousands of yards). At the ranges 
considered in this report this is approximately 2 to 
6 per cent of the range. 

A further British report describes trials with the 
Barr and Stroud Range Finder No. 2 as compared 
with the Infantry Range Finder No. 12 in applica- 
tion to the armored vehicle problem. (545) Tests 
were made of accuracy and speed in fair to good 
conditions, using eight targets and eight range takers. 
Subsequent tests were made in poor light; the targets 
were all ground targets at ranges varying from 935 
to 5,200 yards. Both range finders were mounted on 
their usual tripod mountings. All observers were 
trained in range finding technique. The results 
showed that the error in ranging was almost exactly 
inversely proportional to the base lengths of the 
instruments (80 cm and 100 cm respectively) and 
also that both instruments are almost equal in speed 
of use. It was determined that both instruments were 
equally affected by fading light, giving full accuracy 
until about 10 minutes before the target can no 
longer be seen. There was no measurable correlation 
between the visibility of the target as measured with 
a Casella visibility meter, and the accuracy with 
which its range can be determined, though some tar- 
gets give less accurate readings than others, probably 
because of their shape. The accuracy of the instru- 
ments is discussed in relation to the other errors in 
tank gunnery. The conclusion is reached that the 
100 cm base instrument offers no appreciable advan- 
tage if the accuracy measured in this trial is attained 
on the battle field; if, as seems probable, ranges meas- 
ured in battle are substantially less accurate, this 
conclusion may be incorrect. The average time for 
making coincidence settings was approximately 25 
seconds. 

Another British report indicates the large errors 
found in visual range estimation. (340) Results were 
taken both for estimation of opening range and for 
fall of shot for correction of range. The results show 
that in estimating range, the longer the range the 
greater the error in estimation. The average error, 
without regard to size, rises from 250 to 375 yards 
between ranges of 900 to 2,000 yards, and increases 
rapidly at ranges over 2,000 yards up to 1,000 yards 
error at 2,650 yards. In estimating range correction, 
the larger the correction to be made, the greater the 


USE IN TANKS 


153 


error in estimating it. The use of a sighting telescope 
(powers 2x to 6x) instead of the unaided eye did not 
reduce the errors in estimation of range correction 
whatever the magnification employed. In both esti- 
mation of opening range and for range corrections 
there is a tendency to underestimate long ranges and 
larger corrections. 

In another British report the use of a range finder 
for tank gunnery with immediate fire for effect is 
recommended. (339) It is calculated that in order to 
obtain a minimum standard of one hit in three dur- 
ing fire for effect at a target 6 feet high and 9 feet 
wide at 2,000 yards with a 6-pounder gun, a range 
finder with either no systematic error and a disper- 
sion of observations not exceeding lOr^ (where r is 
range in thousands of yards) or a systematic error of 
12 yards and no dispersion of observations would be 
required. Mention is also made that trained tank 
gunnery instructors are not significantly better in 
visual estimation of range than people with no gun- 
nery training or experience. 

Finally, the British report an experiment to deter- 
mine the suitability of the No. 12 Infantry Range 
Finder (80 cms, 14x magnification) for tank use. (53) 
For this experiment, three instruments were used, 
two mounted on tanks. There were four observers 
ranging from untrained to fully trained men. The 
targets consisted of hull down tanks, mock-up anti- 
tank guns, a hut, and a factory chimney varying from 
668 to 3,784 yards true range. There were six targets 
in all. The range finders stood up well to the vibra- 
tion of a tank in motion. There was no firing. The 
results indicate that for any one target, an average 
observer may be expected to have a systematic error 
of ±10r2 yards and a mean deviation of ISr^ yards 
where r is the range in thousands of yards. It was 
recommended that three readings be taken on a tar- 
get and the median of the three readings be used as 
the range for opening fire. The results indicate that 
even a single range reading gives a satisfactory value 
of the range to be determined, but it is felt that the 
increase in time, probably about 10 seconds, which 
is required to obtain two more readings, would be 
well spent in eliminating the possibility of the gun 
being set to a value determined from a hasty, and 
possibly, erratic, single reading. 

From an analysis of the readings of the individual 
observers, it is evident that the two trained observers 
were much more consistent than either the semi- 
trained or the untrained observer, to a degree of 


almost 50 per cent. All three instruments were found 
to be extremely stable over a temperature range of 
42 to 70 degrees Fahrenheit; in no case did the read- 
ings change more than 3 seconds of arc. 

Studies on Range Estimation 

A similar comparison between visual range estima- 
tion and the use of range finders was made at Fort 
Knox by the Bausch and Lomb Company. The range 
estimation tests are reported along with range finder 
tests. (110) Through the cooperation of the Gunnery 
School at Fort Knox, data were made available on 
the performance of classes in range estimation with- 
out instrumental aid before and after training. The 
group were taken to the site for the range finder 
experiments and estimated the range to several of 
the designated targets without being told the true 
ranges. They were given training once a week for 7 
weeks in range estimations on entirely different 
terrain and then were brought back to the original 
site. These results indicate a marked superiority of a 
series of trained estimates over untrained ones. How- 
ever, these experiments agree with the British find- 
ings that no amount of training will give an average 
accuracy of visual range estimate better than 15 to 
17 per cent of true range. There is some evidence 
that even such accuracy cannot be maintained with- 
out constant and continual practice. And even with 
highly trained observers, individual estimates will 
frequently be made very far from true range. Further- 
more, there will be a tendency for these errors to in- 
crease when a unit proceeds to new terrain and/or 
encounters new or unfamiliar weather conditions. 

These same range estimation results by 60 officers 
before and after training were given further statisti- 
cal analysis by the Princeton Branch of the Frankford 
Arsenal. (244) They find that the percentage error 
in range estimate is independent of true range both 
before and after instruction. The probable error for 
this group before instruction was 30 per cent of true 
range and after instruction this was reduced to 17 
per cent of true range. The percentage of error was 
found to vary, however, not only with range but due 
to the nature of the target and the nature of the 
intervening terrain. 

As a result of this experiment, a member of the 
Fire Control Section of NDRC informally prepared 
a section on training in range estimation which was 
included in the Instruction Manual for Gunnery for 
the Armored Force Schools. 


154 


DEVELOPMENT OF SHORT-BASE RANGE FINDERS 


The Princeton Branch of the Frankford Arsenal 
reports an experimental study of direct and differ- 
ential range estimation without the use of instru- 
ments. (255) Direct estimation is made without the 
aid of known objects at known distances. Differential 
estimation was accomplished when known objects at 
known distances were available to aid in estimation. 
Observation for differential estimation was made 
through binoculars by 50 gunners and tank destroyer 
commanders. The test targets were half tracks at 
ranges from 400 to 2,765 yards. It was found that the 
probable error of a single range estimate by direct 
visual estimation ranges from 17 per cent to 32 per 
cent in different groups of men. A figure of 25 per 
cent is probably representative. On the other hand, 
the probable error of single range estimates by differ- 
ential estimation, determined on the set of 30 half- 
track targets, was 14 per cent in each of the two 
groups of 25 gunners and tank destroyer com- 
manders. It was found that the scatter was relatively 
small where the target was near a reference point 
and increased with increasing separation of target 
and reference point. It is natural to consider that 
errors in range estimation may be of three kinds: 
(1) a tendency of individual men to range consist- 
ently long or short on all targets; (2) a tendency for 
all men to estimate a particular target long or short; 
and (3) to other causes, not associated with (1) or (2) 
above. An analysis of the results indicates that the 
actual errors are a compound from all three sources. 

To determine the relative accuracy of visual range 
estimation and of range finders, two field experi- 
ments were performed by the Bausch and Tomb 
Company at Fort Knox. The purpose of the first 
experiment was highly practical— to determine which 
of several existing instruments should be selected for 
immediate adoption as a range finder for tanks and 
also to determine which sort of instrument and which 
type of field should be further developed for an im- 
proved instrument for this purpose. (108) Six instru- 
ments were available: 

Keuffel &: Esser— 

Invert coincidence (IM, 12x) 

Keuffel Sc Esser— 

Superimposed coincidence (IM, 12x) 

Keuffel &: Esser— 

Stereoscopic reticle (IM, 12x) 

Barr &: Stroud— 

Mark VI (IM, I4x) 


Perkins Sc Elmer- 

Superimposed coincidence (48", 6x) 
Polaroid- 

Stereoscopic with bright line reticle (43", lx) 
Ten targets were selected on rolling terrain at ranges 
from 646 to 5,939 yards and included such realistic 
targets as barns, telegraph poles, sign, tank semi-hull 
down, buildings on distant ridge, an isolated cedar 
tree and finally, a pylon for purposes of calibration 
of the instruments. The subjects consisted of ten men 
from an enlisted detail of Tank Corps personnel se- 
lected from an original group of 25 on the basis of 
standard vision tests. All subjects were untrained at 
the start of the experiment and at no time had knowl- 
edge of results or of true ranges. A total of approxi- 
mately 15,000 readings were taken during a period 
of 3 weeks. 

The raw data were reduced to a common basis so 
that it was possible to compute a “figure of merit” 
for each instrument. It was found that, in terms of 
per cent of error, the three coincidence instruments 
gave the best performance, with the Barr and Stroud 
Mark VI very much better than either of the other 
two coincidence instruments. However, in terms of 
UOE, the Polaroid instrument held an unchallenged 
first place, with the Mark VI a poor second. In this 
connection it will be remembered that the Polaroid 
instrument had the advantage of unit power. As a 
result of this experiment the Fire Control Section 
recommended the adoption of the Barr and Stroud 
instrument as an immediate solution of the problem. 
This recommendation concurred with the opinion 
of the Armored Forces and the instrument was desig- 
nated as the M7. 

Nevertheless, the results obtained with the Pola- 
roid instrument were of great enough interest to 
warrant further study with this type of stereoscopic 
field, utilizing a bright illuminated reticle of the 
Wandermark type. Another reason of importance 
was the finding that one could press the illuminated 
reticle into a material background, such as trees, 
which was not possible with the normal opaque 
reticle of the usual stereoscopic instrument. Hence, 
the Bausch and Tomb Company fitted three stereo- 
scopic Navy Mark 58 instruments for further tests 
at Fort Knox. The first of these was unmodified and 
contained the usual Navy opaque reticle. The others 
were modified so that the reticle of the second was 
an illuminated line and the reticle of the third was 


1 ^RESTRTCTFJi\ 


FIELD COMPARISON OF VARIOUS INSTRUMENTS 


155 


an illuminated star. For comparison with the results 
of earlier tests, the Barr and Stroud Mark VI instru- 
ments was also included. 

Four men of the original test group acted as ob- 
servers and, at the end of the experiment, four men 
of the recorder group also took readings. Only two 
of the original targets could be seen because of 
foliage. In all, seven targets were used, at ranges from 
1,190 yards to 2,793 yards. These consisted of the 
sign and pylon (from the earlier experiment), a hull 
down tank, a tank fully exposed head on, a truck 
with only a small part of the top showing through 
the trees, a truck partly obscured by shrubbery, and 
a bushy tree in the skyline. 

The results indicate that, if equal weight is given 
to the various targets, there is a small, relatively 
consistent performance in favor of the Barr and 
Stroud invert coincidence field over the stereoscopic 
fields considered either from per cent error or UOE. 
Flowever, if the results for a single target with very 
difficult background are eliminated, these differences 
in favor of the invert coincidence field tend to dis- 
appear. Evidence of considerable effect of target and 
background differences were importantly apparent. 
Little difference was seen among the three stereo- 
scopic fields but the illuminated reticles were slightly 
better than the opaque reticle— the illuminated dot 
slightly better than the illuminated line. 

These same four instruments were submitted to a 
factory test at the Bausch and Lomb Optical Com- 
pany. (Ill) A Mark 57 B & L Coincidence instru- 
ment (1 -meter base length) was added to the group. 
Three expert and three novice operators acted as 
observers. Six targets were employed at ranges from 
1,013 to 8,137 yards, exhibiting differences in back- 
ground and conformations both suitable and unsuit- 
able for ranging with both types of instrument. 

The results of this experiment indicate that the 
experts were better than the novices on all instru- 
ments. There is relatively little difference between 
these two classes of observer on the coincidence in- 
strument. There were relatively great differences 
between the experts and the novices with the stereo- 
scopic instruments. There was little difference in 
the results for all of the instruments when used by 
experts. The Mark 58 regular reticle range finder was 
poorest, for targets of this type, for both novices and 
experts. In a short subsequent experiment, to deter- 
mine meteorological effects, the regular and illu- 


minated star reticle instruments were ranged against 
a single target in very bad haze and rain. Under these 
adverse conditions, the star reticle gave considerably 
better accuracy of performance and slightly better 
precision. 

Still another Bausch and Lomb report discusses 
the results of the several Fort Knox experiments. 
(110) This report also considers such general aspects, 
for the choice or development of such an instrument 
for Armored Force use, as ruggedness, stability and 
easy adjustability, convenience of use, portability, 
observer training, and the like. 

Recommendations 

As a result of these various experiments, the Fire 
Control Division of NDRC made certain recom- 
mendations to the Services. (39) (1) A fire control 
system involving the use of a range finder was rec- 
ommended for units of the Armored Force and would 
be extremely useful for other arms of the Ground 
Forces. (2) The Barr and Stroud invert coincidence 
(IM, 14x) seemed to be the best immediately avail- 
able instrument for use in the Armored Force situa- 
tion. (3) In the further development of an instru- 
ment, consideration should be given to a stereoscopic 
instrument of the illuminated reticle type because 
such an instrument could be of extreme value for 
correction of range as well as obtaining initial range 
for opening fire. (4) With the increased accuracy of 
initial range to be expected with the use of a range 
finder, further investigation should be carried out 
to determine the most suitable fire control system. 
(5) With the stabilization of the gun in Armored 
Force units, consideration should be given to the 
development of a complete self-contained fire con- 
trol system with linkage between the range finder 
and the gun. 

Amplifying this last recommendation is a letter 
from the Chief of Section 7.4, NDRC, to the Office 
of the Chief of Ordnance. (579) The scheme contem- 
plates the use of a range finder of stereoscopic type, 
the range knob of which is linked directly to the 
gun sight (or gun sight reticle) so as to introduce 
automatically the proper super-elevation. Two addi- 
tional knobs (or perhaps preferably some form of 
joystick or course-and-speed indicator) are also pro- 
vided. One knob would offset the gun sight by a 
fixed amount in azimuth; the other would offset it 
by a fixed amount in elevation, this latter amount 


156 


DEVELOPMENT OF SHORT-BASE RANGE FINDERS 


being in addition to the super-elevation introduced 
by the range finder. The contemplated procedure for 
this system would be for the tank commander, having 
selected his target, to introduce an azimuth deflection 
through the azimuth knob which, in his judgment, 
would provide the necessary lead to compensate for 
the cross component of the target’s velocity. He 
would also introduce an elevation offset to compen- 
sate for the estimated range component of target 
velocity. The setting of these leads could conceivably 
be made by means of a course-and-speed indicator. 
Once made, these offsets remain fixed until inten- 
tionally changed. The tank commander would then 
range on the target, and the first shot would be fired. 
Through the range finder, which may now be 
thought of as a stereoscopic spotting glass, the com- 
mander would observe the error of the fall of shot. 
This error might be due in part to a false estimate 
of target’s course or speed; it might be in part due to 
inaccurate boresighting, and in part due to such 
things as trunnion tilt, angle of site, and the like. 
Additional leads sufficient to correct it would be set 
into the sight by means of the azimuth and elevation 
knobs. The range finder would again be adjusted to 
present range, and the second round would be fired. 
This process would be repeated until a hit was 
secured. 

The system has been described as if all the func- 
tions were performed by the tank commander. Vari- 
ous distributions of responsibility between gunner 
and commander are possible, and an important part 
of the design problem would be to select the best. 
The essential elements of the scheme are the correc- 
tion of all those factors which can be expected to 
change but slowly with time through the medium 
of fixed azimuth and elevation deflections; and the 
correction of the one remaining factor, range, which 
can be expected to vary rapidly with time by means 
of direct coupling from the range finder knob. 

A fire control system for the Gun Motor Carriage 
T 70 is proposed by the Princeton Branch of the 
Frankford Arsenal. (250) Tank targets, either mov- 
ing or in full defilade at ranges up to 2,000 yards, 
are of the greatest tactical importance and the pres- 
ent fire control system does not provide sufficient 
accuracy to accomplish hits with the first or early 
rounds fired. Simple range estimation has proved 
to give errors of at least 25 per cent at least 40 per 
cent of the time. Actual tests of differential range 
estimation (i.e., in terms of known ranges to other 


objects in the field of fire, when these ranges have 
been previously determined by survey or with a 
range finder) give an improvement in accuracy over 
unaided range estimation which is discouragingly 
slight. Survey methods (determination of range by 
measuring angles at two or more separated points) 
are slow and subject to uncertainty in target identi- 
fication and difficulty in the communication of data. 

Hence the fire control scheme envisages the utili- 
zation of the invert coincidence range finder M9 for 
the determination of range and the Princeton Branch 
have devised a special mount for its use. On the ve- 
hicle, the instrument would be mounted on the 0.50- 
caliber antiaircraft gun ring. The mount and range 
finder may be quickly and easily removed for off- 
vehicle operation and, in this case, a unipod is pro- 
vided as a support. Binoculars M3 are collimated 
with the range finder and brow pad, eyeguard, and 
soft rubber eyepieces are provided. In both on- and 
off-vehicle operation the commander determines the 
range and announces to the gunner. Other aspects 
of the fire control system need not be developed here. 

The theoretical expected performance of a hit on 
first round with the proposed system on a stationary 
target as compared with the present method at 2,000 
yards is: for head on tank (2x2 yard target) 0.16 as 
against 0.4 with range estimation and 0.06 with range 
card; for broadside-on (2x6 yard target) 0.32 for 
proposed system as against 0.09 for visual estimation 
of range and 0.16 for use of range card. The compari- 
son of the probability of hitting on the first round 
and of hitting a tank in hull defilade {\/^ x 2 yard 
target) at 2,000 yards is 0.045 by proposed system as 
against 0.015 with visual estimation of range and 0.03 
with use of range card. Data are also given for es- 
timated performance against moving tank targets 
and, at 2,000 yards, show ratios of improvement of 
the proposed system to the present system in ratios of 
approximately 3 to 1. 

Australian Range Finding Sight 

For Armored Force application, a range finder 
sight system with linkage to the gun was developed 
by a group in Australia. This is fully described in a 
British report which also records non-firing trials 
with the system by the British Tank Armament Re- 
search Committee. (546) The instrument was de- 
signed to transfer rapidly and automatically the 
range measured by the range finder to the moving 
reticle of the sighting telescope in terms of tangent 


RESTRICTED 



TANK FIRE CONTROL 


157 


elevation. An 80-cin base range finder (Infantry No. 
12 Mark IV) is coupled to a X3 episcopic telescope, 
provided with a moving reticle. The range finder 
and telescope employ a common eyepiece, a moving 
prism allowing the optical path to be transferred 
from one instrument to the other. Both the rotation 
of the prism and the coincidence setting of the range 
finder are controlled through flexible drives and a 
milled knob situated conveniently for the gunner. 
The operation of setting the range finder into coin- 
cidence moves the horizontal crosswire of the sight- 
ing telescope into such a position that laying it on 
the target will automatically impart the correct range 
table tangent elevation for the range in question and 
the gun and ammunition. Throughout the present 
tests, the actual range in yards was unknown to the 
operators. 

Results show that the mean deviations obtained 
with the Australian range finding sight were about 
one and one-half times as great as with Infantry 
Range Finder No. 12 and that the Australian sight 
failed in fading light slightly before the Infantry 
instrument. The accuracy of the transfer of range 
from the range finder to the telescope was adequate. 
The Australian range finding sight showed a net 
saving of about 10 seconds over the conventional 
range finder and fixed reticle sighting telescope in 
finding the target, measuring its range, and laying 
the gun. It is noteworthy that the mean deviations 
in range determination increased, with both range 
finders, by a factor of 3.5 times, when the operators 
knew they were being timed. It was also noted that 
the halving of the range finder sight lost its adjust- 
ment during cross-country runs of about 3 miles. 

Firing trials with the Australian system are given 
in another T.A.R. report. (547) These report a com- 
parison of the Australian system experimentally 
mounted on a Centaur Tank and the ordinary No. 
39 Mark 1 telescope with fixed reticle in a Cromwell 
III tank. Both vehicles mounted a 6 pr 7 cwt H.V. 
gun and H.E. Mark I.T. ammunition was used 
against targets representing hull-down tanks at 
ranges between 800 and 1,500 yards. Trials were car- 
ried out in hazy winter daylight. Four experienced 
tank commanders and four gunners familiar with the 
ordinary and the Australian methods were used in 
the trials. The results show that the Australian 
range finding sight offers a saving of at least 3i/2 
rounds in number of rounds and at least 2 i /2 minutes 
in time spent in bringing the target within the 90 


per cent range zone of the gun, when compared with 
the normal method of visual estimation of range 
and use of a fixed reticle. On four target pairs the 
number of rounds necessary for a hit were, for the 
Australian range finder sight: 6, 4, 1, and 7 as against 
the normal method of 16, 8, and no hit after 10 and 
12 rounds. However, T.A.R. point out that the 
Australian system would be difficult to fit into exist- 
ing tanks and would require extra holes in the front 
or side armour and that it could not be used for 
turret-down fire. Certain modifications of the system 
are suggested, such as the introduction of a range 
scale so that the information could be passed to other 
tanks and adjustment for different types of ammuni- 
tion and different muzzle velocities. 

Range Finder Fields 

The results outlined above indicate that there may 
be a considerable effect of the type of field used when 
ranging is against indistinct targets of the ground 
type, characteristic of Armored Force or Ground 
Force combat. Hence the Princeton Branch of the 
Frankford Arsenal attacked this problem of range 
finder held experimentally. The hrst report deals 
with the precision and consistency for three observers 
using eight helds on three targets for a total of about 
5,400 readings. (241) The helds compared were: 

1. Superimposed coincidence 

2. Erect coincidence 

3. Invert coincidence, ortho motion 

4. Invert coincidence, pseudo motion 

5. Invert ortho-pseudo 

6. Erect ortho-psuedo 

7. Ortho stereo, with reticles 

8. Pseudo stereo, with reticles 

The experiments were performed on the Eastman 
Trainer, using vectograph photographs of ground 
targets. The results again indicate that there are 
differences in relative performance of the various 
helds with different targets. Hence an analysis was 
also made using the tank targets. 

The results of this analysis indicate that the super- 
imposed coincidence held appears dehnitely inferior 
in precision and consistency. Erect ortho-pseudo ap- 
pears inferior in precision, though its consistency 
performance is good. Pseudo-stereo is inferior in con- 
sistency. The invert coincidence ortho has the best 
relative position of the eight helds for both consist- 


RESTRICTED 

I II ■■ — ^ 


158 


DEVELOPMENT OF SHORT-BASE RANGE FINDERS 


ency and precision. No results for accuracy were ob- 
tainable by this method. A description of the various 
helds is included. 

In another experiment, performed by the Prince- 
ton Branch of the Frankford Arsenal, a comparison 
of fields was made with the Mark 4 Trainer again 
employing the same three vectographic targets. (242) 
This comparison was primarily between invert- 
foreground and invert-sky combinations. The targets 
were presented in monocular, split-field invert co- 
incidence and binocularly in ortho-psuedo stereo, 
using the foreground- and sky-invert in both cases. 
The results indicate that averaged over all four ob- 
servers, no significant differences were found between 
coincidence and ortho-pseudo presentation or be- 
tween invert sky and invert foreground for this sort 
of ground target. However, individual observers 
showed marked differences in performance with 
respect to type of presentation. 

The Princeton Branch of the Frankford Arsenal 
reported an analysis of fire control design for A.P.C. 
projectiles. (246) This is a theoretical study of vari- 
ous factors which may affect this problem. It pre- 
sents, in both tabular and graphical form, the re- 
quired accuracy of ranging to obtain a given accuracy 
of trajectory location and the expected accuracy of 
various range finding devices. The graphs and tables 
of this memorandum are intended to serve as a basis 
for estimating the probable efficiency of range find- 
ing devices and for comparing different instruments 
of this sort. Methods are given for translation into 
comparable unit instruments which may vary in base 
length, magnification, and corrections as a result of 
different weather conditions. Data regarding ranges 
for different degrees of elevation for the M61 and 
M62 are given. The study concludes with the state- 
ment that visual estimation of range is only satisfac- 
tory at ranges well below 1,000 yards when using 
these projectiles. 

The Princeton Branch has performed some experi- 
ments and has entered into a theoretical discussion 
to study the problem of sensing shots from the 76-mm 
M 1 gun mounted on motor carriage T 70. (249) They 
conclude that present sensing accuracy in regard to 
fall of shot is insufficient to take full advantage of the 
inherent accuracy of the gun. One suggestion for the 
improvement of this sensing accuracy might be the 
use of a range finder as a sensing instrument. The 
M9 Range Finder was tested, but as it stands, is not 
well adapted for use as a sensing instrument. The 


investigators believe it is possible that a range finder 
such as the T16E1, in which the upper half of the 
field is duplicated, might be of some value in this 
regard. 

15 2 SOME STUDIES OF SIMULTANEOUS 
TRACKING AND STADIOMETRIC 
RANGING 

Simultaneous Hand and Foot 
Operation 

At the request of the Naval Bureau of Ordnance, 
the Foxboro Company did several experiments on 
simultaneous tracking and stadia ranging for small 
caliber antiaircraft guns. (221) The first was a field 
study involving the ranging and tracking of 431 
actual crossing flights, of which 71 were rejected 
because of photographic recording difficulties. The 
study involved the measurement of over 15,000 
frames of moving picture film. Three types of stadio- 
metric reticle were used: an illuminated ring, an 
illuminated disc and six illuminated pointed dots. 
Tracking was done by the usual handle bar assembly 
and ranging was accomplished by pressure on a foot 
pedal with both feet. The results show that there are 
no signficant differences between these three types 
of reticle so far as ranging accuracy is concerned, but 
that the ring is definitely inferior to the dots and disc 
in tracking accuracy. These conclusions hold for a 
wide variety of sun and sky conditions during the 
tests but with twilight and dark conditions excluded. 

It was found that the task of ranging and tracking 
simultaneously is difficult for the inexperienced 
Naval operators used in this test, who tend to con- 
centrate now on one and again on the other aspect 
of the operation. Both variability from test to test 
and the ratio of poorest to best operators are greater 
in ranging than in tracking, indicating that ranging 
is more difficult. No evidence of improvement was 
found during the tests due to the limited amount of 
practice afforded. But it is very probable that oper- 
ators would show definite improvement in these two 
simultaneous operations, especially in ranging, with 
training and considerably more practice. 

In these field tests, the majority of operators 
tended to range short, that is, to make the diameter 
of the reticle larger than the largest dimension of 
the target. Less than 10 per cent of all measured 
frames showed the opposite tendency. The largest 



SIMULTANEOUS TRACKING AND RANGING 


159 


ranging errors were found with larger reticles or 
shorter ranges both because of the increasing rate of 
change in target velocity and because of greater diffi- 
culty in matching the larger extents visually. 

Although the tests were made primarily to obtain 
stadia ranging data, information regarding types of 
tracking errors was obtained from viewing films of 
the tests as well as from measurements of the frames. 
Rough quantitative checks of moving pictures of the 
tests revealed tendencies toward different constant 
elevation errors with the three reticle patterns. With 
ring, the tendency is to center the target in the upper 
half of the circle while with the disc, the tendency 
is to center in the lower half. With the six dot pat- 
tern, which was the only one having a center dot, 
there was a tendency to center the target in the 
upper half but the elevation errors were much 
smaller, and the total time off target was much less 
than with the other two patterns. Since total tracking 
error as shown by measurements of the individual 
frames was equal for dots and disc, it would appear 
that the center dot is of value in reducing elevation 
error rather than azimuth error. 

For regular flights the per cent ranging error for 
the average of all operators was 45 for ring, 47 for 
dots, and 39 for disc while the tracking errors in mils 
were 15.6 for ring, 7.6 for dots, and 7.4 for disc. In 
certain special courses of accuracy of ranging in 
flights across the sun, the ranging errors in per cent 
were increased to 66 for ring, 83 for dots and 60 for 
disc while the tracking errors in mils was increased 
to 26.0 for ring, 15.3 for dots and 13.6 for disc. The 
method of obtaining the dot pattern is described 
and illustrated in the text. 

These results and the magnitude of the errors 
obtained were not encouraging from the point of 
view of accuracy of fire. However, it will be remem- 
• bered that the operators used in the first field experi- 
ment were not trained or experienced in these simul- 
taneous operations. Hence the Foxboro Company 
subsequently did a laboratory experiment to estimate 
the feasibility of training a man to operate simul- 
taneously triple controls for following a target in 
azimuth, elevation, and range. (221) A hand and 
foot technique was used in which azimuth and ele- 
vation were tracked by varying combinations of side- 
to-side and up-and-down movements of wide handle 
bars while an activity like ranging was achieved by 
pressing with the feet on either side of a pivoted 
cross-bar. In an additional experiment, a pair of 


opposed-action pedals were substituted for the cross- 
bar. Throughout the experiments the tracking and 
stadiometric ranging were accomplished while view- 
ing a circular target through a reticle having a center 
dot and hexagon pattern of variable diameter. 

The results with both the crossbar and opposed 
pedal forms of foot control demonstrate that under 
favorable conditions in 2 or 3 hours considerable 
proficiency is possible in simultaneous hand and foot 
functioning. All operators were successful in devel- 
oping this coordination. The simultaneous function- 
ing, however, reduces the accuracy of separate man- 
ual and pedal functioning. Even under favorable 
conditions, 10 hours of practice (100 runs) may be 
required to compensate the added difficulty of the 
triple performance. More accurate simultaneous 
functioning followed 4 hours preliminary practice 
on a single function than 4 hours in the simultaneous 
function. Under such circumstances, 5 or 6 hours of 
simultaneous functions were sufficient to compensate 
for the added difficulty. During the simultaneous 
performance, greater improvement was in the rang- 
ing when the initial training was only in ranging; 
it was in the tracking when the initial training was 
only in tracking. A significantly greater advantage 
was secured in the case of tracking, presumably be- 
cause of the significantly greater difficulty of the 
tracking operation. 

The ranging scores with the crossbar and with the 
opposed-pedal foot control did not differ significantly 
in accuracy. The pedals, however, may be rated 
slightly higher on three counts— less foot slippage, 
slightly better score and unanimous operator prefer- 
ence. It is pointed out that the foot-hand controls of 
the sort investigated are open to criticism because 
the coordinations might break down under strain or 
distraction and because the feet are often needed to 
support or brace the body during manipulations of 
hand controls. 


2.2 All-Hand Operation 

The Foxboro Company reported another study on 
simultaneous tracking and stadiometric ranging. 
(226) This is a comparative study of all-hand con- 
trols, by handle bar tracking in azimuth and eleva- 
tion plus twisting motorcycle grips for ranging versus 
handle bar tracking with the hands plus double 
pedal ranging with the feet. The results of 21 subjects 


f RESTRIcfEr) 1 


160 


DEVELOPMENT OF SHORT-BASE RANGE FINDERS 


indicate that operators may learn either type of con- 
trol for the simultaneous triple operation. The re- 
sults indicate the same order of accuracy in the all- 
hand performance as in the hand-foot performance. 
This result was true for highly practiced operators 
as well as for a new group having no previous train- 
ing in either tracking or ranging. The hand ranging, 
while hand tracking, did show slightly higher accu- 
racy than the foot ranging, while hand tracking, but 
the difference did not prove to be significant. Learn- 
ing curves are given and the new operators demon- 
strated marked practice improvement during 5 hours 
of simultaneous tracking and ranging with either 
type of control. Also, changing the sensitivity of the 
hand ranging control, by a factor of two, gave rise 
to no significant change in accuracy either of the 
ranging itself or the simultaneous tracking. 

15.2.3 Single Versus Double Pedal 

A final Foxboro Company report in this series 
contrasts results with three types of foot ranging 
controls while simultaneously tracking in azimuth 
and elevation with handle bar controls. (227) The 
three types of foot control used were opposed double 
pedal and two accelerator type of right foot pedal 
actuated by pressure against a spring, in one case 
hinged at the instep and in the other case hinged at 
the heel. Springs of three degrees of stiffness were 
used in each of the accelerator types of foot control, 
giving mean operating pressures in pounds from 
1.5 to 11.5 for the central hinged pedal and 3.5 to 

18.3 for the heel hinged control. In all three cases, 
pressure of the right foot increased the diameter of 
the stadia ranging reticle, which was of the dot pat- 
tern. Eight trained and six untrained observers acted 
as subjects, the latter being put through 7 hours of 
practice with each type of control. 

The results show that no significant difference in 
accuracy was revealed for simultaneous direct handle 
bar tracking and foot pedal ranging regardless of 
whether the foot pedal ranging was accomplished 
with two feet on double opposed pedals, with one 
foot on a single spring pedal hinged under the instep. 


or with one foot on a single acceleration-type of pedal 
pivoted at the rear of the heel. For the trained ob- 
servers, who had previously been practiced in double 
pedal operation in a former experiment, showed a 
greater accuracy of 5-10 per cent for this type of con- 
trol. No significant effects on pedal operation re- 
sulted from the sensibly obvious changes in spring 
resistance employed. Finally, no consistent operator 
preferences were reported either with respect to 
pedal type or to spring tension. For every operator 
and for every type of ranging control, there was 
greater ranging error while simultaneously tracking 
than when ranging alone. 

Conclusion 

These several studies on simultaneous tracking 
and simultaneous ranging have been gathered into 
a single report attached to a Report to the Services 
issued by the NDRC Fire Control Division. (42) 
The following recommendations are made. 

1. Inasmuch as no significant differences were dis- 
covered for several types of ranging controls while 
doing simultaneous handle bar direct tracking in 
azimuth and elevation, it is safe for designers of such 
instruments to introduce motorcycle grip, opposed 
two foot pedal or spring opposed single pedal con- 
trols into new designs depending upon the position 
of the operator and/or the space restrictions present. 
It may be noted, in the case of a highly unstable 
platform, that the single foot control may be prefer- 
able to the use of both feet because, in this case, the 
operator could then firmly plant his unused foot for 
greater support and stability. 

2. The disc pattern reticle for sights of the type of 
the Mark VII should be given further field test with 
trained Service personnel. 

3. Operators should receive very considerable 
training in simultaneous azimuth and elevation 
tracking and stadiometric ranging before they are 
called upon to use this difficult operation in combat. 
The preliminary training should start with 3 to 4 
hours tracking alone before stadia ranging is added 
to obtain the best results in the shortest time. 


RESTRICTED 


Chapter 16 


RETICLE DESIGN 


161 INTRODUCTION 

T he problExM of reticle design was in the minds of 
a number of NDRC investigators early in their 
contracts. One early study was made at Tufts College 
in an effort to produce a simpler reticle pattern 
which might even give better accuracies and preci- 
sions than the standard types. (562) Another impor- 
tant aspect was that the proposed design would be 
less affected by target position. The reticle consisted 
merely of the outline of a circle subtending 34 min- 
utes of arc as the fiducial mark. This circle should 
determine a fiducial plane against which the target 
should be ranged. The results showed that such a 
reticle helps, in some cases, to overcome the target 
position effect when the operator is instructed to 
range by keeping the target in the center of the circle. 
Ranging on a stationary target, the accuracy of two 
of the six subjects, with the target placed in the 
middle of the circle, was significantly better than it 
was with the under-circle or above-circle positions. 
One observer had a larger constant error in the 
middle-of-circle trials than he had in either of the 
above or below trials. 

Some early experiments on reticle design were 
conducted at Brown University. For this purpose 
they developed a special apparatus (126) for the 
comparison of stereoscopic settings with different 
reticles. The apparatus, which is fully described in 
the text, allows for rapid change of the reticle pat- 
terns which are presented to the subject and for a 
maximum of reproducibility in reticle position rela- 
tive to the target. Illustrative results are given for five 
reticle patterns: Navy Post and Navy Diamond, 
standard Army, a reticle in which the fiducial mark 
consists of two concentric circles in a larger cross- 
hatched field all in the same plane (Riggs reticle), 
and a photographic reticle showing two chimneys 
above a building as the fiducial marks. No data are 
given with this last reticle pattern. These are also 
described in a second report. (119) 

The preliminary data for three subjects show the 
following rank order: Navy Post, Navy Diamond, 
Army, and Riggs reticles. The differences, however, 
are not very great. The report emphasized in the 
analysis that day to day variations in subjects are 


ordinarily large enough to swamp differences in the 
settings which may be obtained with different reticles 
under proper conditions.' This low consistency of 
these results was probably due to changes of criteria 
of contact and it is emphasized that further work 
on reticle design should probably initially empha- 
size aids in maintenance of uniform criteria of judg- 
ment of stereo contact. 

The Full-Line Reticle 

Several reports from the Princeton Branch of the 
Frankford Arsenal are concerned with experiments 
dealing with a new suggested form of reticle pattern. 
These are called full-line reticles and consist of one 
or more vertical lines extending across the entire 
field. (260) It is hoped that these full-line reticles 
may reduce the bad effects of errors in height adjust- 
ment between fiducial mark and target. Such reticles 
with one and two lines were tested against standard 
reticle in the M6 Stereoscopic Trainer. This instru- 
ment provides (1) targets of fixed range, (2) targets 
moving in range without tracking errors, (3) targets 
moving in range with tracking errors in azimuth or 
elevation, and (4) targets moving in range with track- 
ing errors in both azimuth and elevation. The single 
line reticle consisted of a single line running verti- 
cally and centrally across the whole of the apparent 
field. This line had an apparent width of approxi- 
mately 0.5 mils or 8 UOE. The double line reticle 
consisted of two similar lines parallel with a separa- 
tion of 30 mils— the combination being centered in 
the field. Simulated airplane targets in different 
shapes, sizes and attitudes were used. Different posi- 
tions of reticle and target were employed. Observa- 
tions were made by two practical subjects. The results 
indicate that, under these conditions, the perform- 
ance of the full-line reticles was not inferior to the 
standard reticles and it is recommended that further 
consideration be given full-line reticles as a means 
of reducing the ill effects of height-adjustment error. 

A second report from the Princeton Branch of the 
Frankford Arsenal gives further results of work with 
reticles of the full-line type. (261) The single line 
reticle and a double full-line reticle with a separation 


RESTRICl FI 


161 


162 


RETICLE DESIGN 


of 125 mils and the standard reticle were used in the 
M6 Steroscopic Trainer. Two sets of vectograph 
targets were used — one selected for good and the 
other for bad height adjustment. The same two ob- 
servers were used. Only the variability of the median 
settings is reported. The results indicate that no 
clear or consistent superiority of any of the three 
reticle types was apparent for any one target. How- 
ever, it appears that there were statistically signifi- 
cant differences between the relative performance of 
the different reticles on the targets in height adjust- 
ment as compared with those out of height adjust- 
ment. Using the geometric mean as a measure of 
average performance, the standard and single line 
give larger errors when the target is out of height 
adjustment while the double line gives slightly better 
results under these conditions. The investigators 
point out that, while the differences between these 
values are statistically significant, it is not clear that 
much practical importance is to be attached to them. 
The level of variability obtained was very largely 
dependent on the particular targets and vectographs 
and it seems unsafe to generalize too far. ft would 
seem especially dangerous to apply the results to 
range finder practice without considerable addi- 
tional evidence. However, it is interesting to note, 
subject to the limitations above, that on the whole 
the standard reticle performs consistently well in 
both in and out of height adjustment series. 

The problem of reticle design seemed of sufficient 
importance so that a systematic program on this topic 
was initiated at Brown University. This program 
was not particularly designed to pick out the best 
reticle of the existing patterns or of any other pat- 
erns which might be devised, but rather sought to 
determine the good and bad principles for the design 
of reticle patterns. 

16.1.2 Imperfections in the Reticle Field 

A preliminary series of Brown University studies 
investigated the question of the effect of imperfec- 
tions in the reticle field of stereoscopic range finders. 
This was an extremely practical problem because in 
1943 and earlier it was reported that manufacturers 
were experiencing difficulty in producing a sufficient 
supply of perfect reticles. It was subsequently under- 
stood that this difficulty was overcome completely, 
though at the cost of substantial numbers of rejects. 


However, at that time, it seemed advisable to study 
the problem of reticle inspection as well as the possi- 
ble effects of small spots or imperfections on the reticle 
field. The first Brown study had to do with the relia- 
bility of reticle inspection. (161) A series of 30 reticle 
blanks, showing various degrees of blemish, were 
examined by five members of the regular inspection 
staff of a manufacturer. The same group was retested 
with the same blanks 2 months after the initial exam- 
ination. The results demonstrate great differences in 
judgments among inspectors in both test and retest. 
Thus, although an inspector may be reasonably, 
although not extraordinarily consistent with herself, 
she may nevertheless be completely out of line with 
other inspectors. For example one inspector rejected 
30 per cent of the blanks on the first test and 20 per 
cent of the same blanks on retest; while another ob- 
server rejected 86.7 per cent and 73.3 per cent of the 
same blanks on test-retest respectively. 

The second report from Brown University deals 
with an experimental study of the effects of imper- 
fections intentionally introduced into the reticle 
field. (162) These consisted of opaque circles of 
different sizes and, in every case, these intentional 
imperfections were of considerably greater magni- 
tude than any which would ever be encountered in 
a field instrument. In this experiment only settings 
on fixed targets were employed. The results indicate 
that the presence of such imperfections result in a 
very slight and statistically unreliable decrease in 
precision of stereoscopic setting and a probable de- 
crease in the consistency of such settings. There is 
some slight evidence, again statistically unreliable, 
that the extraneous stimuli in the reticle field influ- 
ence settings in the direction of giving “short” read- 
ings. However, the general conclusion of this report 
is that the presence of imperfections of the magni- 
tude introduced has no great effect on stereoscopic 
settings. 

In order to determine if these findings would be 
confirmed if both stereoscopic movement and track- 
ing errors were introduced into the ranging situa- 
tion, Brown University did an additional dynamic 
study which is described in a third report. (174) 
Under these more exacting conditions of observa- 
tion, when imperfections, for which fusion was both 
possible and impossible, were introduced and with 
stereoscopic movement and tracking errors also pres- 
ent, it was found that the presence of the extraneous 
forms in the reticle field produces no appreciable 




RESTRICTED 


RETICLE IMPERFECTIONS-RETICLE PATTERN TESTS 


163 


effect on the precision or accuracy of ranging. 

These three Brown University studies form the 
supporting data of a Report to the Services issued by 
the Fire Control Division of NDRC. (40) This report 
makes the following recommendations: (1) That 
greater uniformity of inspection and resulting saving 
of materials would probably be effected by manu- 
facturers’ introduction of objective quality control 
systems, such as use of specimens with known degrees 
of imperfection for comparison by inspectors of both 
reticle blanks and of finished reticles. (2) The using 
Services should inform stereoscopic range finder ob- 
servers, both in training and in the field, that slight 
imperfections which may appear in the reticle field 
should be ignored because they will not affect the 
quality of range finder performance. 

Another special study was made by Brown Uni- 
versity on a comparative study of the design of in- 
ternal adjuster targets for range and height finders. 
These studies (139, 142, 143, 158, 159) are attached 
to a Division 7 NDRC Report to the Services. (31) 
These reports are described in the section on Cali- 
bration of the fnstrument in this present summary 
and are mentioned in this place solely for the sake of 
completeness because they deal with the design and 
pattern of a reticle target but on a type of problem 
quite different from the one presently under discus- 
sion. 

16 2 EXPERIMENTS ON RETICLE 
PATTERNS 

We now turn to the description of the several 
experiments reported by Brown University in their 
systematic study of the design of reticles for stereo- 
scopic range and height finders and of the determina- 
tion of the systematic principles underlying the vari- 
ous kinds of patterns which might be designed. The 
first set of experiments dealt solely with opaque 
reticle patterns. The apparatus employed in many 
of these experiments is adequately described and 
illustrated. (170) The instrument was especially 
developed for the purpose of comparing designs pro- 
posed for use in the reticle field of a stereoscopic 
range finder. The instrument makes possible the 
rapid exchange of one reticle design for another so 
that several designs may be used successively during 
the course of a single experimental period. The 
reticles and targets are provided by large photo- 
graphic plates. The size of the plate is such that the 
common photographic flaws of specks, scratches. 


pinholes, and the like are not discernible by the 
observer. The left and right reticle plates for each 
design are permanently cemented side by side to a 
common backing of plate glass, thus assuring a con- 
stant disparity between corresponding points in the 
field. The targets are provided with narrow, vertical 
slits which may be superimposed upon the central 
lines of the reticles for purposes of establishing a true 
zero error of ranging. No optical system of projection 
or collimation is involved, since reticles and targets 
are in essentially the same physical plane. Small 
variations of azimuth and elevation of the target can 
be introduced in random fashion by a mechanical 
system which simulates typical tracking errors. Ran- 
dom variations in depth can also be introduced and 
the task of the observer is to maintain stereoscopic 
contact between the target and the reticle by the use 
of a range knob. 

16.2.1 Approximating Zero Error 

The method for approximating the point of zero 
error or true range in ranging on artificial stereo- 
scopic targets within a reticle field had been previ- 
ously described in a Brown University report. (138) 
The essential feature of the method is the provision 
that the distance between target images for the left 
and right eyes shall be the same as the distance be- 
tween the central fiducial lines of the left and right 
reticles. This is accomplished by observing the right 
field monocularly and establishing an optical coin- 
cidence between a fixed point on the reticle and a 
corresponding point on the movable right image of 
the target. These corresponding points are so placed 
that, when they are in coincidence, the target images 
are automatically of the same disparity as the center 
post images for the two eyes. The coincidence can be 
established with a high degree of precision and with 
small variation from observer to observer. Hence the 
method is useful for measuring accuracy of perform- 
ance in a stereoscopic research instrument as well as 
precision and consistency which could be obtained 
without its use. Experiments with six subjects indi- 
cate that the monocular coincidence observations 
have a greater stability, with respect to both preci- 
sion and agreement among observers, than do stereo- 
scopic observations made under similar conditions. 
The mean of the individual values for average devia- 
tion is 0.74 UOE for stereoscopic observation and 
only 0.36 UOE for monocular. 


RESTRICTED 


164 


RETICLE DESIGN 


Types of Course Used 

Another report of the Brown University group 
discusses certain methodological considerations hav- 
ing to do with the type of course used in experiments 
on reticle design and the reliability of the observers’ 
determinations. (172) These experiments are con- 
cerned with an analysis of two factors which are rep- 
resented in the reticle pattern experiments but not 
in military range finding: (1) the effects due to the 
use of a random course and, (2) effects due to the 
manner in which the observers’ records are obtained. 
The first series of experiments, here reported, dem- 
onstrated that the basic “random movement’’ rang- 
ing course employed does not give results greatly 
different from those given by a course comparable to 
those encountered under field conditions in anti- 
aircraft work. The course respresenting field condi- 
tions is essentially a “sine wave” course which gives 
an apparent change in range, first through an in- 
crease and then through a decrease for a crossing 
course. Further experiments are concerned with six 
conditions of target presentation. These were: (1) 
random and erratic stereoscopic movement with 
simulated tracking errors; (2) no stereoscopic move- 
ment but simulated tracking errors in which the 
observer throws the target off after making contact 
and makes a new setting; (3) random stereoscopic 
movement but no tracking errors; (4) sine wave 
stereoscopic movement with tracking errors; (5) 
stationary target with target nose placed directly 
under 5 minutes below center post of reticle and no 
tracking errors; (6) stationary target with nose 5 min- 
utes below and 47 minutes to right of the center post 
of the reticle and no tracking errors. The Navy line 
reticle was used and 10 experienced observers each 
made 50 range readings for each condition. The 
results indicate that the introduction of stereoscopic 
movement caused a statistically reliable decrease in 
the precisions of making range settings. The effects 
of tracking errors on precision are not great by com- 
parison with effects of stereoscopic movement. The 
mean precision scores for each condition in UOE 
were: (1) -3.58; (2) -3.28; (3) -3.44; (4) -3.44; 
(5) -3.13 and (6) -2.95 UOE. 

16.2.3 Factors Affecting Precision 

In still another experiment the Brown University 
group ascertained the reliability of readings made by 


different observers when the random movement 
course was used. Small differences in readings were 
shown among the three experienced observers em- 
ployed. However, the differences were well within 
the limits of accuracy which may be expected in the 
field situation when due consideration is given, e.g., 
to the range transmitter problem. An experimental 
design which will absorb variances due to observer 
differences in readings is suggested in the report. 
The average discrepancies for constant error or accu- 
racy were 0.50 UOE and for precison were 0.33 UOE. 

A preliminary experiment at Brown University 
compared the precisions of settings made with four 
present standard Service reticle patterns. (160) These 
were the Navy Solid Diamond, the Navy Open 
Diamond; the Navy Line and the Army. Observa- 
tions were made with timed intermittent readings on 
a stationary target by eight practiced observers di- 
vided into four groups of two observers each. The 
experiment was so designed that each group of ob- 
servers viewed the reticles in a different sequence, 
a procedure aimed to balance out effects due to prac- 
tice and fatigue. The mean values for precision, ex- 
pressed as average deviations from individual mean 
settings in seconds of arc are: Navy Line: 19.2; Army: 
19.5; Navy Solid Diamond: 22.2; and Navy Open 
Diamond: 23.8. The factor of reticle design makes 
a contribution— significant at the 5 per cent level— 
to the variance of all the average deviation values. 
The slight superiority of the Navy Line reticle evi- 
denced in these experiments is in line with a similar 
finding of a previous experiment when bracketing 
ad lib was allowed. The present experiments do not 
provide any data on what might happen under con- 
ditions of continuous contact. 

Fore and Aft Marks 

The Brown University group attacked the prob- 
lem of fore and aft marks and of their position. The 
results are given in two reports. (163, 165) The first 
of these experiments is concerned with a preliminary 
comparison of the effect of apparent position of fore 
and aft reticle marks on the precision of stereoscopic 
settings. Results were obtained on 1 1 reticle patterns, 
with fore and aft marks at various combinations of 
7.5, 15, 30, 60, and 120 UOE from the control line 
of Navy Line reticle. A reticle with no fore and aft 
marks was used for comparison. The experiment was 
so designed that the five experienced observers 
viewed the reticles in different sequences. Readings 


FORE AND AFT MARKS 


165 


were made at 10-second intervals on a fixed simulated 
aerial target. The fore and aft marks were presented 
singly at the different apparent distances and in con- 
secutive pairs at the different apparent distances 
from the fiducial line. It was found that the smallest 
average deviations are yielded by reticles in which 
single pairs of fore and aft marks are located at 30 
to 120 UOE from the central line. The poorest pre- 
cision was given by a reticle with single pairs of fore 
and aft marks at 7.5 UOE. Reticles with other com- 
binations of fore and aft marks lie between the ex- 
tremes. A reticle with no fore and aft lines gives next 
to the poorest precision. The precisions of the differ- 
ent combinations in rank order in seconds of arc are: 
30 UOE: 14.9 seconds of arc; 120: 15.2; 60: 15.4; 120, 
60, 30, 15 and 7.5. All present: 16.7; 60 and 30: 17.3; 
15: 19.0; 15 and 7.5: 19.1; 30 and 15: 19.4; 120 and 
60: 19.4; no fore and aft marks: 21.4 and 7.5: 22.3. 

These results indicate that highest precision is 
obtained with reticles having single pairs of fore and 
aft marks 30 UOE or more from the central lines. 
In order to determine whether the single-pair fore 
and aft marks are reliably better than other combi- 
nations, the investigators performed an analysis of 
variance in which the three single-pair reticles which 
had the highest rank orders were contrasted with all 
others. The results of this statistical treatment show 
that observer to observer variance is significant at 
the 1 per cent level and that variance within the two 
classes of reticles is significant below the 1 per cent 
level. Hence the investigators conclude, as a general 
principle, that single pairs of fore and aft marks 
disposed at sufficiently great distances from the cen- 
tral line, provide advantageous conditions for mak- 
ing stereoscopic settings. Fore and aft marks placed 
too near the central line or too many fore and aft 
marks in any reticle pattern seem to be disadvantage- 
ous, at least for ranging on fixed targets. 

The second Brown University experiment dealing 
with the positioning of fore and aft marks in the 
reticle field is described in a second report. (165) 
In the previous experiment, these marks were so 
varied in separation as to provide the impression of 
perspective lines crossing at the central post. In the 
present study, the investigators are concerned with 
precisions with fore and aft reticle marks which vary 
in apparent distance from the central line but which 
maintain the same lateral separation throughout the 
range in depth. Single pairs of fore and single pairs 
of aft marks were disposed at the following stereo- 


scopic UOE distances from the central line; 7.5, 15, 
30, 60 and 120. The lateral distance between reticle 
lines in each pair was constant at a separation cover- 
ing 13 per cent of the distance between the two posts 
adjacent to the central post in the central line of the 
Navy Line reticle. As seen in depth, the fore and aft 
marks seemed to be disposed symmetrically— in the 
lateral plane— about the central post of the reticle. 

The results show that precisions for fore and aft 
marks lying at 7.5, 15, and 30 UOE from the center 
are, as an average for five subjects, about 16.5 sec- 
onds of arc. The precision decreases for the lines at 
60 and 120 UOE to between 25 and 30 seconds of 
arc. The decrease in precision shown for the two 
later reticles was accompanied by the qualitative 
finding that fusion of the reticles was impossible for 
all subjects under these conditions. Hence, from the 
point of view of reticle design, it would seem that 
fore and aft fiducial marks at great distances from 
the center line should be disposed with wide separa- 
tions in the sterescopic plane. 

Differences in Configurations 

A more elaborate experiment was planned by the 
group at Brown University to answer a number of 
questions: (1) Do changes in “psychological configu- 
ration” of reticle pattern cause changes in precision 
of stereoscopic setting? (2) Does length of fiducial 
line influence precision of setting? (3) Does precision 
of setting with a horizontal fiducial mark differ from 
precision of setting with a vertical fiducial mark? 
(4) What is the influence of “peripheral mass” on 
precision? (5) What is the influence of lateral separa- 
tion of full-line fiducial marks on precision? (6) What 
degree of precision is obtained with two “miscellane- 
ous reticle configurations”? (168) 

The same apparatus and 1 1 experienced observers 
were used at different times and on different groups 
ranging with a total of 17 reticle patterns. Stereo- 
scopic settings on fixed targets simulating aircraft 
were made at timed 10-second intervals. The experi- 
ment was divided into three series. The philosophy 
underlying the first series started with the hypothesis 
that the function of a reticle is to define a plane in 
space. From this point of view, it is important to 
know whether certain types of configurations, by 
their “psychological relationships”, define planes 
more adequately than certain other kinds of con- 
figurations. Complete analysis of this problem would 
require determinations on an infinite number of 


^estricTted^^ 


166 


RETICLE DESIGN 


configurations and hence is not practicable. Never- 
theless, it is of some value to determine whether 
differences in conhgurations lead to differences in 
precisions of ranging. For example, does a particu- 
lar reticle show greater internal cohesiveness in de- 
fining a plane than another type of configuration? 
Reproductions of the reticles used in this and the 
other series are given in the text. Five reticle designs 
were used in the first series: (1) A reticle in which 
there is an insistence of long vertical lines with breaks 
in the center. (2) A symmetrical radiating pattern, 
filling most of the visual field, in which the fiducial 
marks are short vertical lines, 21 in number. (3) A 
variety of fiducial marks consisting of a short central 
vertical line surrounded by four different letters of 
the same height. (4) A pattern in which opportunity 
is given to enclose the target by a geometric figure 
inasmuch as the central line is enclosed by a rectangle 
with two other vertical lines placed laterally outside 
this area. (5) This is similar in principle to reticle 
(4) and is a modificaton of the circle reticle to fulfill 
the conditions of target position. In this reticle, seg- 
ments of a circle are joined by vertical lines enclosing 
a short central line. In all of these patterns the length 
of the central line is the same and the nearest sur- 
rounding lines are at the same distance. Also, the 
target was always placed in the same relative position 
laterally and below the central line. 

An analysis of variance of the results on the pre- 
cision values shows that variance due to observers is 
significant at the 1 per cent level and variance due to 
reticles is not significant at the 5 per cent level. Ex- 
cept for the fifth reticle, which gave exceptionally 
poor precisions, the reticles of this series do not result 
in great differences in precision of performance. 
These results would indicate that emphasis on such 
principles as numerous radiating fiducial marks fill- 
ing a large part of the field; dissimilarity of fiducial 
marks; the provision of geometrical figures within 
which the target appears; and the provision of long 
lines do not, under the present conditions, give re- 
sults which indicate tremendous superiority of one 
pattern over another. Hence, rather than emphasiz- 
ing further work on configurational factors, it would 
seem that a more fruitful approach would be along 
the line of examining reticles which provide enough 
comparison fiducial marks in such aspects that the 
operator is given a maximum opportunity to make 
precise settings, when due consideration is given to 
tracking errors. 


The second series of reticles contain four patterns 
designed to test the influence of length of line in 
precision of setting. These have the following forms: 
(1) Full single line to left so that target may be cen- 
tered in the field close to the line; (2) Short vertical 
single in same position; (3) Single full vertical single 
line with break just below center of field; (4) Short 
vertical single line with break in similar position. 
Two other reticles were constructed to allow for an 
evaluation of precisions obtainable with vertical as 
contrasted with horizontal lines, the target appear- 
ing in comparable spaces between the lines. For this 
purpose two reticle patterns were developed; (5) A 
single vertical line through the center of the field 
with a larger break than (3) above, and (6) two 
parallel horizontal lines separated by the same 
amount as the break in (5). 

The results indicate, by an analysis of variance for 
the class long-line reticles versus the class short-line 
reticles that variance due to subjects is significant at 
the 5 per cent level and variance due to class is sig- 
nificant also at the 5 per cent level. These are results 
with fixed targets and it may well be that the longer 
length of line may be valuable in giving opportunity 
to the observer to make precise settings with changes 
in the position of the target if tracking errors are 
present. In the second comparison in this series of 
experiments, the results show that vertical lines give 
better precisions than horizontal lines— comparison 
of reticles (5) and (6)— and that this difference is 
valid and reliable at the 1 per cent level. This differ- 
ence was markedly present for four of the five ob- 
servers used in this part of the experiment. 

In a third series of experiments, the results of 
which are given in this same Brown University re- 
port, six more reticles were developed to answer cer- 
tain specific questions. They are considered in pairs. 
Reticles (1) and (2) present full-line configurations, 
the target appearing in comparable positions be- 
tween the lines. Reticle (1) is a full line reticle with 
six evenly spaced vertical lines across the entire field. 
Reticle (2) has exactly similar lines but there have 
been added two large dark masses at the top and 
bottom of the field. It has been suggested that the 
presence of such masses might better define the plane 
to be used for ranging against the target as well as 
leading to better fusion of this plane in space. The 
results indicate, however, that reticle (2) does not 
give better precision than reticle (1) and hence the 
result does not justify the conclusion that a “pe- 


) RESTRICTED^ 


RETICLE CONFIGURATIONS 


167 


ripheral mass” enhances precision of setting except 
in the cases of two of the ten observers used. 

Reticles (3) and (4) were constructed to test the 
influence on precision of a series of broken vertical 
lines, disposed throughout the stereoscopic field. 
Reticle (3) consisted of 7 broken vertical lines evenly 
spaced. Reticle (4) had 21 similar broken lines 
evenly spaced but obviously closer together than the 
separation between lines in reticle (3). The results 
show that reticle (3) with the wider separation of 
lines was reliably better than subject precision with 
reticle (4) at the 5 per cent level. This was true for 
eight of the ten observers. Subject observations indi- 
cate that reticle (4) is a poor configuration. Fluctua- 
tions in localization of this reticle occurred very fre- 
quently due to “horizontal slip” or the fusing of un- 
paired lines. All subjects agree in this observation. 
Thus it seems that parallel vertical lines in a reticle 
pattern should be displaced from one another 
through such a distance that equivocal fusion is 
avoided. More will be said about horizontal slip later 
in this section. 

Reticles (5) and (6) present two miscellaneous 
designs. Reticle (5) is the closest approximation to 
the design of the German R40 instrument described 
by the Frankford Arsenal. (228) It consists of three 
small solid diamonds with two smaller rectangles 
between evenly spaced in a horizontal line. Reticle 
(6) consisted solely of a circle placed in the middle 
of the field. The results for reticles (3), (4), (5), and 
(6), as tested by analysis of variance indicate that 
variance due to observers is not significant at the 5 
per cent level but that variance due to reticles is sig- 
nificant at the 1 per cent level. The rank order of pre- 
cisions is (6) (the circle reticle); (3) (widely spaced 
vertical lines), (5) (the German design) and (4) (the 
narrowly spaced broken vertical lines). 

From this whole series of experiments, the in- 
vestigators draw the following conclusions on con- 
figurations to be used in reticle design. (1) Some 
reticles which differ considerably in configurational 
aspects do not give great differences in precision of 
stereoscopic setting for fixed targets and without 
tracking errors. (2) Long-line reticles seem to be 
slightly preferable to short-line reticles. Length does 
not seem to be a variable of first-order significance 
but, nevertheless, the evidence indicates that long 
lines lead to slightly better precisions than short 
lines. Vertical lines are preferable to horizontal lines. 
Probably the choice of long line will be strengthened 


as tracking errors are introduced. (3) The results do 
not justify the hypothesis that a peripheral mass in 
the fiducial field enhances the precision of stereo- 
scopic setting. (4) When vertical lines are used in 
the reticle pattern, their separations must be great 
enough to exclude the possibility that unpaired lines 
in the two eyes may be fused, hence placing the fidu- 
cial plane at the wrong place in space. 

Two additional experiments with fixed targets are 
described in another Brown University report. (173) 
The first experiment compares the effects, on pre- 
cision of stereoscopic setting, of two reticle patterns: 
the Navy open diamond and a so-called “three-dot” 
pattern. This latter consists of three small dots ar- 
ranged in a triangle with the dimensions slightly 
smaller than those of one of the Navy diamonds. The 
three dots are arranged in a triangle whose base ap- 
pears to the observer’s right with the vertex to his 
left. The dot at the vertex appears closer by 2.5 UOE 
than the dots at the base. The task of the observers 
is to obtain stereoscopic contact of an airplane target 
between the single nearer dot and the more remote 
pair of dots. Five observers were used. It was found 
that the precisions for the two reticles were not re- 
liably different, being 9.3 seconds for the Navy open 
diamond and 10.2 for the three-dot pattern. These 
differences are not significant at the 5 per cent level. 
A second experiment compares precisions with a 
single Navy open diamond and the three-dot pattern. 
The precision obtained for the three-dot pattern was 
10 seconds and for the single diamond 7.4 seconds. 
These differences are not significant at the 5 per cent 
level. 

Stereoscopic Movement and Tracking Error 

The next several experiments are based upon ob- 
servations made with both stereoscopic movement 
and tracking errors introduced. The first of these 
dynamic experiments contrasts the observations on 
standard Service reticles or on such reticles modified 
to a certain extent. (171) Six reticles were used of 
which four were of standard pattern: (1) Army; 
(2) Navy line; (3) Navy solid diamond and (4) Navy 
open diamond. The other two were modified Army 
reticles as follows: (5) Army without nearest and 
farthest fore and aft marks and (6) Army with all 
fore and aft marks removed. Settings were made by 
ten well trained observers. Settings were made at 
timed 10-second intervals on a moving airplane tar- 
get undergoing slow changes in depth and exhibiting 


RESTRICT 


168 


RETICLE DESIGN 


tracking errors. The experiment was so designed that 
the ten observers viewed the reticles in different bal- 
anced order sequences. 

The mean values of precision, expressed in UOE 
were found to be, in rank order: Navy solid diamond: 
4.17; Navy line: 4.23; Navy open diamond: 4.26; 
Army without extreme fore and aft marks: 4.39; 
standard Army: 4.41; and Army without any fore and 
aft marks: 4.44. In this experiment, involving a tar- 
get which exhibits changes in range and tracking 
errors, the factor of reticle design makes a very slight 
and statistically insignificant contribution to the 
variances of all precision values. Variances due to 
observers are significant at the 1 per cent level, while 
variance due to days is not significant at the 5 per 
cent level. Thus, the reticle patterns used seemed to 
have little influence on precision of setting. The 
same general finding applies to the accuracy of the 
settings. For accuracy, variance due to reticles and 
days is negligible, while variance due to observers 
is signihcant below the 1 per cent level. Hence one 
may conclude that of the six reticle patterns ex- 
amined, no pattern gives significantly more accurate 
or more precise settings than any other pattern. 

Comparison of Reticle Patterns 

In another memorandum is reported an extensive 
experiment by the Brown University group in the 
dynamic situation. (176) The first series of observa- 
tions is concerned with an analysis of precisions of 
stereoscopic settings made by ten well-trained ob- 
servers on 17 reticle patterns. Conditions involved 
a random course of stereoscopic movement plus 
tracking errors. The subject could control height of 
image by a fine height adjustment knob. The reticle 
patterns are described and figured in the text. Many 
of the patterns are those previously described above 
in the present summary. The 17 reticles used differ in 
regard to such aspects as number of lines, size of 
central gap, anchoring or lack of anchoring of the 
lines to the circumference of the visual field, presence 
or absence of a peripheral mass and other details. 
The experiment was designed so that the ten ob- 
servers viewed the reticles in a balanced order. 

It was found that the five reticles yielding the best 
precisions in this series are as follows in rank order: 
the captured German reticle from the R40 — 3.39 
UOE; a single vertical line with a 1.8 degree gap in 
the center of the field— 3.43; two banks of short lines 
with a 0.9 degree gap— 3.55; seven vertical lines with 


a 1.8 degree gap— 3.57; seven vertical lines with a 
3.6 degree gap— 3.61 UOE. None of these reticle pat- 
terns had any fore and aft marks. An analysis of 
variance performed on all 17 reticles shows that 
reticle differences are not significant at the 5 per cent 
level. Analysis of variance on the seven-line reticles 
anchored to the circumference of the field also indi- 
cates that differences due to size of gap are not sig- 
nificant at the 5 per cent level. This involves com- 
parison of five patterns with no gap (full-line) and 
gaps of 0.9, 1.8, 3.6, and 7.2 degree. Similar results 
were obtained with the single vertical-line group of 
reticles and the unanchored seven-line reticles when 
imperfect control of height of target image was pres- 
ent for the largest gaps. A control experiment, with 
perfected height adjustment indicates, for the seven- 
line anchored reticle, that precision of performance 
for the 7.2 degree gap is not significantly different 
from precision for the 1.8 degree gap. 

Analysis thus indicates that, so far as precision is 
concerned, there is little to choose between a seven- 
vertical-line reticle and a single vertical-line reticle 
with the mean position of tracking errors in the 
center of the field. With off-center tracking errors, 
the seven-line reticle, in providing more points of 
reference, should undoubtedly give better precisions 
because of less separation between the target and a 
fiducial line. 

It was found that the presence of large peripheral 
masses at the edge of the field does not improve 
precision over what may be obtained without a peri- 
pheral mass— 3.83 and 3.63 UOE respectively. The 
German R40 reticle ranked high in precision. How- 
ever, it was insignificantly better, statistically, than 
most of the other reticles. Anchoring of reticle lines 
to the boundaries of the visual field has no advantage 
over short lines which are not anchored and, indeed, 
the values for these two classes are identical at 3.72 
UOE each. Finally the correlation between observ- 
ers’ preference ratings for reticles and precision of 
performance was low (r = 0.32). 

In a second series of experiments described in this 
report, nine reticle patterns were used in an attempt 
to answer three questions: (1) Does thickness of 
reticle lines have an influence on stereoscopic per- 
formance? (2) How good is the German R40 reticle 
and do modifications influence its goodness? (3) Are 
circle reticles satisfactory? To answer these questions 
nine reticle patterns were constructed: (1) Seven-line 
reticle with thin lines of 1.85' width; (2) Same reticle 


[ RESTRICTE^ 


SENSITIVITY TO LOSS OF STEREOSCOPIC CONTACT 


169 


with lines thickened by a factor of 5.0 to 9.25' width; 
(3) Navy line reticle (at 14x) with no fore and aft 
marks; (4) Same reticle but with center post thick- 
ened by a factor of 5; (5) German R40 reticle with 
one set of fore and aft marks; (6) Navy open diamond 
reticle (at 14x) with nearest sets of fore and aft marks 
only; (7) German R40 reticle standard with no fore 
and aft marks; (8) Outline circle reticle and (9) 
Gray circle reticle with density of circle about 35 
per cent. 

Twelve subjects were used in this experiment and 
analyses were made with respect to precision, accu- 
racy, and subject-to-subject consistency. The viewing 
conditions involved a random course stereoscopic 
movement and tracking errors. Five height of target 
image adjustments were under control of the ob- 
servers. 

The results indicate that thickness of line does 
not seem to contribute reliably to either precision 
variance or accuracy variance. The thick line reticles 
give better subject-to-subject consistency than the 
thin line reticles. The R40 German reticle, with or 
without fore and aft marks, is not superior with re- 
spect to precision to the Navy open diamond reticle, 
with or without fore and aft marks. For accuracy, 
the German reticle, with one set of fore and aft 
marks, is superior to the Navy diamond reticle, with 
one set of fore and aft marks. The enemy reticle 
with fore and aft marks is superior to itself without 
fore and aft marks in respect to accuracy. It also 
gives excellent subject-to-subject consistency. Finally, 
the circle reticles, both circle and disc, are unsatis- 
factory. They give poor precision, high constant 
errors, and low subject-to-subject consistency. They 
are extremely susceptible to height-break errors. 

16.2.4 Problems of Reticle Design 

Loss OF Contact with Reticle 

A number of special studies were performed at 
Brown University dealing with specific problems of 
reticle design. The first of these reported an experi- 
ment to determine the ability of stereoscopic ob- 
servers to signal loss of contact of a target with the 
reticle in a dynamic situation. (175) This is essen- 
tially the situation of aided ranging on a dive target. 
It is of considerable interest to learn something of 
the limits of response made by a stereoscopic observer 
to an accelerating change from stereoscopic contact to 


a condition where the target appears in front of or 
behind the reticle. For example, let it be assumed 
that a device is available for aided ranging. So long 
as the device exactly compensates for the change in 
disparities produced by the target, the target will 
appear to be in stereoscopic contact with the reticle. 
However, when the device does not properly predict 
the rate of change of disparity, the target will appear 
to move away from the reticle at a rate which will 
be determined by the speed and range of the target. 
The problem is particularly important in the case of 
approaching diving targets, where, when the aiding 
device does not compensate for the rate of change of 
disparity, the target will appear to emerge in front 
of the reticle. It is conceivable, in the latter circum- 
stance, that range finder operators might perform 
some operation which would change the aiding rate 
and thereby improve the firing data. The feasibility 
of such operations depends on many factors, among 
which is the sensitivity of the operator to loss of 
contact with the reticle. If sensitivity to loss of con- 
tact is poor, then it would seem that this fact would 
set important limitations to the type of operations 
for which the instruments could be used. 

With these considerations in mind, the Brown 
University group made observations on the just per- 
ceptible parallax difference for loss of stereoscopic 
contact and, in particular, for a condition where the 
target is emerging from contact with the reticle in 
an accelerated manner. It will be recognized that 
these are indeed most exacting conditions of ob- 
servation. 

The experiment was performed on the Eastman 
Trainer and simulated an airplane diving at the rate 
of 200 mph from 3,000 to 1,000 meters. At the mo- 
ment when the observer saw the plane at a range 
different from that of the reticle he pressed a button 
on the fine height adjustment knob which was con- 
nected electrically to a recording pencil. Thirteen 
subjects were used and all were practiced in conven- 
tional ranging methods but were not skilled in re- 
acting to loss of contact. The results indicate that 
the just noticeable difference in parallax angle, 
under these conditions, is both very large and vari- 
able. The investigators believe, on the basis of this 
result, that a method which would involve a correc- 
tion to true range, based on the observers’ response 
to a rapid deviation from stereoscopic contact, would 
be unsatisfactory. Such corrections, made for dive 
conditions, would occur with routine errors of the 


RESTRICTEib 



170 


RETICLE DESIGN 


order of about 10 to 30 UOE. The duration of the 
reaction times during which the target is advancing 
to these values from loss of contact with the reticle 
vary from about 2 to 8 seconds depending on the 
rate of range change of the target. Reaction time is 
fairly rapid for short ranges and slow for long ranges. 

Absence of Fine Elevation Adjustment 

A second Brown University report on a special 
problem is concerned with stereoscopic performance 
of different reticles in the absence of fine elevation 
adjustments. (177) All previous experiments had 
been performed under conditions allowing the ob- 
servers to use the fine elevation adjustment. Since 
some Service instruments are not equipped with a 
fine elevation adjustment system, it seemed worth 
while to compare a sample of reticles, previously 
compared under other conditions, in the situation 
duplicating what would be encountered in a range 
finder with no fine elevation system. Presumably the 
fact that observers would then be unable to control 
tracking errors of elevation would introduce poorer 
precision values, because the target would deviate 
considerably from the reticle marks. This considera- 
tion would be especially pertinent if the reticle were 
of the Navy diamond or line variety. However, if 
vertical lines were used which ran across the whole 
reticle field, it might be possible that the lack of fine 
elevation adjustment would not interfere with high 
precision. 

To test this hypothesis, four different reticle pat- 
terns were compared for precision under these con- 
ditions. They were: (1) standard Navy diamond; 

(2) seven vertical full lines; (3) seven vertical lines 
with a central gap of 1.8 degrees; and (4) a similar 
full-line reticle with a gap of 7.2 degrees. The full- 
line reticle gave the highest precisions while the 
Navy diamond and the vertical lines with the largest 
gap gave the poorest precisions. The values in UOE 
are: full-line reticle— 2.71; line with 1.8 degree gap— 
8.83; Navy diamond— 3.29; and line with 7.2 degree 
gap— 3.37. A dynamic course with random change of 
stereoscopic distance and random tracking errors was 
used throughout. However, the differences among 
reticles are not statistically reliable. A comparison 
of precisions for the Navy diamond reticle under 
conditions of fine elevation adjustment or no adjust- 
ment yielded a statistically unreliable superiority 
for the condition when the elevation adjustment was 
present— the values being 3.34 and 3.65 UOE. 


False Fusion 

Another special problem on false fusion in various 
reticle patterns is reported by Brown University. 
(178) Theoretically it is possible for a stereoscopic 
observer, under certain conditions, to establish con- 
tact between a target and a reticle at more than one 
reading on the range scale. This situation may de- 
velop when, by “horizontal slip” on the part of the 
observers’ eyes, unpaired vertical lines of the reticle 
fuse to produce a reference range different from the 
true one. Theoretically several false ranges may be 
established for reticles having lines of the central 
row too close together. Several reticles were devised 
to demonstrate this phenomenon. All consisted of 
parallel vertical lines with a break in the center of 
the field. The patterns were (1) seven such broken 
vertical lines; (2) similar but with 11 parallel lines; 

(3) similar but with 21 parallel lines close together; 

(4) similar with 21 lines but with five small open 
circles above and below the break on appropriately 
paired lines; (5) similar to reticle (4) but with black 
instead of open circles. 

The results indicate that observers may make set- 
tings which are quite in line with theoretical expecta- 
tions derived from a consideration of amounts of slip 
associated with distances between adjacent fiducial 
marks of a reticle. Such slip may occur even when 
adjacent unpaired reticle lines have dissimilar but 
not incompatible configurations, as in the case of 
the last two reticles. In certain cases observers give 
readings indicative of 1 degree of slip, or fusion of 
adjacent unpaired lines, and at other times, readings 
indicative of fusion of unpaired lines separated by 
two intervals. How serious such a situation might be 
in the field is shown, for example, for the Army 
Height Finder which has a lateral separation between 
adjacent lines in the reticle of 60 mils of apparent 
field at 24x. False fusion or horizontal slip of 1 degree 
would here produce a range error equivalent to 
1.014 UOE, if one assumes that one is dealing with 
a 4-meter range finder at 24 power and with a target 
at a range of 10,000 meters and with the range scale 
reading 10,000 meters. The convergence angle for 
these conditions is 164.8 UOE. It would be possible, 
with horizontal slip to fuse unpaired posts of the 
reticle so that, by moving the range knob, stereo- 
scopic contact could be established for the same target 
at a new value of convergence angle of 1,178.8 UOE 
(1,014 -|- 164.8). Under these conditions and for the 


EFFECT OF RANGING ERRORS 


171 


same target, the range scale would read 1,400 meters. 
It is probable that because of the great difference in 
range involved and the fact that the fore and aft 
marks would not be fused for the 1,400-meter range, 
a confusing situation of this sort would not normally 
exist in Service instruments. Nevertheless, considera- 
tion should be given to this possible source of error 
in reticle design, particularly as it applies to reticles 
lacking fore and aft marks, as in the case of full line 
reticles or their modifications. Reticles with lines 
closely spaced in the central row provide the greatest 
opportunities for establishing successive reference 
planes as a result of increased horizontal slip. Hence 
no reticle should have central lines which are too 
closely spaced. Such false fusion can be largely 
avoided by the use of characteristic identifying marks 
on paired lines. Finally, the presence of fore and aft 
marks may avoid false fusion because only a fusion 
condition can lead to the proper appearance of the 
entire reticle field. 

Height Adjustment Errors 

An important report from Brown University deals 
with the effect of height of image adjustment errors 
for different reticles. (179) The first experiment in 
this series deals with the effects on stereoscopic per- 
formance of differences in elevation of the two visual 
fields under conditions where corresponding ele- 
ments in the two fields maintain constant relative 
positions. The left field is elevated or depressed 
through various degrees of angular movement and 
the subject makes stereoscopic settings for a target 
which exhibits stereoscopic movement and tracking 
errors. Precisions and constant errors of subject per- 
formance were analyzed as a function of the differ- 
ences in elevation existing between the two visual 
fields. The results for seven trained observers using 
the Army reticle demonstrate the movements— up to 
zb 3 mils— of the left visual field with respect to the 
right field do not result in poor ranging performance, 
providing no changes in the relative positions of 
corresponding elements in the fields occur. In these 
experiments antiaircraft targets and a simulated 
sky background were employed. 

The situation is quite different if there is target 
height break or poor height adjustment of the target 
images themselves. A preliminary experiment was 
performed with 13 observers and 11 reticle patterns 
and a more extended experiment using 8 trained 
observers and 8 reticle patterns. Errors of height 


adjustment of target image of 0, 0.75 and 2.0 mils 
were used. The results indicate that, for height of 
adjustment errors in excess of those encountered in 
an instrument adequately adjusted by trained per- 
sonnel, reticles which present more complex figures 
such as diamonds, fore and aft marks do not increase 
or decrease the effectiveness of performance in the 
presence of height adjustment errors. For example, 
the Navy diamond reticles were most affected by a 
2-mil height break error and a single vertical line 
was least affected. An analysis of variance performed 
for the 2-mil error indicates that, at this separation 
of images, variance due to reticles is significant at the 
1 per cent level while observer variance is significant 
below this level. This result may be understood if 
one considers that misalignment of the target images 
leads to the introduction of varying disparities in 
the eyes when a given fiducial mark is of such shape 
that the horizontal distance between target and fidu- 
cial mark changes when the target is moved vertically 
with respect to the fiducial mark, as in the case of the 
diamond. 

Thickness of Reticle Design 

Brown University, having determined the im- 
portance of height of image adjustment errors (see 
179) reported elsewhere in this summary, did a fur- 
ther experiment on this problem using four reticle 
patterns. (182) These consisted of (1) the Navy 
closed diamond without fore and aft marks; (2) three 
widely spaced vertical lines; (3) five evenly spaced 
vertical lines; and (4) five vertical lines unevenly 
spaced. The last is a new pattern not tried before 
in the Brown experiments. It was designed to mini- 
mize the possibility of false fusion without providing 
fore and aft marks because it is impossible for an 
observer to fuse unpaired lines on the two eyes with- 
out observing either a doubling of the number of 
reticle lines or a view of a reticle in which fiducial 
lines appear in different depth positions. The results 
of the present experiment are in harmony with those 
of the previously reported results in showing that 
height of adjustment errors have influence on stereo- 
scopic performance only when the errors are extreme 
and at least greater than 1 mil. Of the four reticles 
tried, the Navy diamond gives the poorest perform- 
ance for the largest height break of 2 mils. The long 
vertical-line reticles are not so susceptible to height 
of image break. Although these tendencies are clear 
cut, none of the differences are statistically significant. 


RESIRTCTED 


172 


RETICLE DESIGN 


Brown University presents an experiment con- 
cerned with an analysis of the performance of stereo- 
scopic observers when observations are made on a 
stationary target parts of which are obscured by 
reticle lines of various thicknesses. (181) A series of 
preliminary experiments demonstrated that, when 
thick reticle lines obscure parts of the target, ob- 
servers tend to develop unwanted, complex forms 
of ranging performance. Ranging under these con- 
ditions involves use of lateral movement and coin- 
cidence cues. When, in the final experiment depen- 
dence on unwanted cues was minimized by a 
procedure which did not allow for bracketing, 
performance with thick-line reticles, which obscured 
large parts of the target, was poorer than perform- 
ance with thin-line reticles. In these final experi- 
ments nine trained observers used single vertical 
full-line reticles 4 minutes 3 seconds and 1 degree 22 
minutes in width. The target subtended an angle of 
5 degrees 8 minutes. The average standard deviations 
of settings with the thick-line was 9.44 UOE and with 
the thin line 1.94 UOE. These differences are statis- 
tically reliable. 

Torsion of Eyeballs 

Another phase of reticle design, investigated at 
Brown University, was the possible effects of torsion 
of the eyeballs. (183) Torsion, the rotation of the 
eyeballs about their sagittal or antero-posterior axes, 
is a phenomenon which occurs in all persons, with 
or without known eye defects. It occurs presumably 
because the external eye muscles have their posterior 
attachments nasal to rather than directly behind the 
eyeball. Such torsion increases with increase in the 
degree of convergence and with increase in elevation. 

Consideration of the range finder situation leads 
to the belief that very little torsion occurs in viewing 
the visual field of the instrument. Nevertheless, it 
seemed worth while to examine the possibility that 
torsion might influence stereoscopic settings because: 
(1) Long, vertical reticle lines might show a slightly 
greater separation of the images in the two eyes at 
the tops of the lines than at the bottoms. Thus the 
tops of the reticle lines might seem further away than 
the bottoms. (2) Targets seen below the reticle might 
seem slightly nearer than targets seen above the 
reticle. (3) Since torsion may be overcome by the 
provision of strong fusional stimuli, observations 
made with “peripheral masses” in the visual field 
would be expected to show diminished torsion effects. 


if such should occur. 

In the experiments, no significant effects on stereo- 
scopic settings could be traced to the presence or 
absence of peripheral masses and these are believed 
unnecessary. The results did indicate that there are 
great individual differences in the differences be- 
tween above reticle and below reticle settings. Of the 
eight observers tested, very few show comparable 
readings for the above and below conditions. Most 
of the subjects gave greater range settings when the 
target was below the target but the reverse tendency 
appears with the remainder. 

Brown University reports still another experiment 
testing a five-vertical-line reticle with respect to re- 
sistance to height break. (184) This reticle was pre- 
sented in its simplest form and in modifications in- 
volving presence or absence of fore and aft marks or 
of a horizontal line which was placed below the 
fiducial marks. The results show that the vertical line 
reticle shows high resistance to height break and also 
it is possible to add fore and aft marks and/or a 
horizontal line to the reticle without detriment to 
observer performance. 

In another section of this report is discussed the 
question of precision and accuracy of height of image 
adjustment for the two reticle patterns with fore and 
aft mark and with and without the horizonal line. 
Two types of target were used— a poor target with 
fuzzy edges and the clear airplane target used in 
previous experiments. Both monocular and binocu- 
lar methods of adjustment were employed. No differ- 
ences were found between the two methods. So far 
as single settings are concerned, it was found that 90 
per cent of the individual readings of all subjects 
with the poor target fall within a spread of 1 mil from 
zero error (defined as the average for the monocular 
series). With tracking errors introduced in the poor 
target and with the binocular method, which is a 
condition which gives poorest precisions, 77 per cent 
of the individual readings of all subjects fall within 
1 mil of the zero error setting given by the monocular 
method. Higher percentages of readings fell within 
the 1 mil limit for all other conditions. Finally, it 
was found that the introduction of a short horizontal 
line into the reticle pattern does not add appreciably 
to the precision of making height adjustments. 

All of these Brown University reports on opaque 
reticle patterns are summarized in a final report. 
(186) The authors are led to believe, from their ex- 
periments, that the most important considerations 


RESTRICTED \ 


CONCLUSIONS 


173 


with regard to reticle design center about the prob- 
lems of height of adjustment errors and false fusion, 
i.e., the fusion by the observer of unpaired reticle 
lines. 

Conclusions 

In regard to the height of adjustment errors, it 
seems safe to conclude that vertical line reticle con- 
figurations provide better insurance than do others 
against poor performance when the height of target 
above or below the reticle is unequal in the two eyes. 
For height of adjustment errors either in excess of 
those encountered in an Army instrument ade- 
quately adjusted by trained personnel (±0.5 mil of 
apparent field) or probably within conventional 
errors exhibited by Navy personnel under conditions 
which routinely involve difficult conditions of rang- 
ing, reticles which present simple vertical line fidu- 
cial marks are superior to those which present more 
complex figures, such as diamonds. The vertical lines 
of the reticle should be sufficiently long, of 5' of 
true field. The vertical lines should not be so thick 
as to obscure large parts of stationary targets and 
0.20' of true field is recommended. 

The question of false fusion is important to reticle 
design because, unless a reticle is so designed that 
fusion of unpaired reticle lines is made impossible, 
an observer might in certain instances establish con- 
tact between a target and a reticle at more than one 
reading of the range scale. For reticles without fore 
and aft marks it would be necessary to calculate a 
proper spacing for vertical marks of the central line 
so that false fusion may be eliminated. Under certain 
circumstances, this procedure may lead to a very few 
widely spaced lines at high magnifications. For this 
reason, it may be desirable, in some reticles, to in- 
clude a minimum number of fore and aft marks. An- 
other type of reticle, one with varying interline 
spaces, may also be used to eliminate the possibility 
of false fusion. It is the experience of the investigators 
that the presence or absence of fore and aft marks of 
the conventional design has little effect on stereo- 
scopic performance. Their presence eliminates the 
possibility of false fusion but a minimum of such 
marks is all that is required. The report recommends 
that there should not be more than a single pair each 
of fore and aft marks and that 25 to 50 UOE may be 
reasonably recommended as a satisfactory spatial 
separation in depth from the central line. 

The report contains recommendations for the pat- 


tern of a Service stereoscopic reticle previously re- 
ported to the Navy Bureau of Ordnance at their 
request. (185) This consists of five vertical fiducial 
lines, with one pair each of fore and aft marks, all of 
a size and in an arrangement which best takes into 
account the configurational principles derived from 
the experiments. 

This final report from Brown University including 
19 Memoranda reporting individual experiments, 
all of which have been summarized above, is attached 
as supporting data to a Report to the Services issued 
by the Fire Control Division of NDRC. (44) This 
report recommends Service trials of the reticle de- 
sign suggested by the Brown University personnel. 

163 STADIOMETRIC RANGING 

The following reports from the Foxboro Com- 
pany are discussed at this point mainly for the sake 
of completeness because the work was done under the 
direction of the NDRC Fire Control Division, and 
also because the material is of considerable interest. 
These experiments dealt with different patterns of 
reticles for stadiometric ranging and of different 
reticle patterns of sights rather than optical range 
finders of the more conventional sorts. 

The first Foxboro report retails comparative tests 
of three stadia ranging reticles with ring, dots and 
disc patterns. (218) Tests were taken under field 
conditions with actual airplane targets and with high 
angular velocities. A photographic technique of re- 
cording was employed. Tracking was done with 
handle bar controls and ranging with a double foot 
pedal. Acting as observers were 13 Naval personnel 
and one civilian, none of them had previously had 
training with handle bar control or those types of 
sights for simultaneous tracking and ranging. The 
results indicate that there are no significant differ- 
ences between these three patterns so far as ranging 
accuracy is concerned but that the ring is definitely 
inferior to dots or disc in tracking accuracy. 

An examination of the records indicates that the 
task of ranging and tracking simultaneously is diffi- 
cult for inexperienced operators who tend to con- 
centrate now on one, now on the other aspect during 
the test. Both variability from test to test and the 
ratio of poorest to best operators are greater in rang- 
ing than in tracking. No evidence of improvement was 
found during the tests due to the limited amount of 
practice afforded. In these tests the majority of op- 


174 


RETICLE DESIGN 


erators tended to range short, that is, to make the 
diameter of the reticle larger than the largest dimen- 
sion of the target. Less than 10 per cent of all meas- 
ured photographic frames showed the opposite tend- 
ency. Largest ranging errors were found with the 
large reticles or shorter ranges, both because of the 
increasing rate of change in target velocity and be- 
cause of greater difficulty in matching the larger 
extents visually. 

Although the tests were made primarily to obtain 
stadia ranging data, information regarding types of 
tracking errors was obtained from viewing the films 
of the tests as well as from measurement of the indi- 
vidual frames. With the ring reticle the tendency is 
to center the target in the upper half of the circle, 
while with the disc the tendency is to center the lower 
half. With the dot pattern, which was the only con- 
figurationhaving a center dot, there was a tendency to 
center the target in the upper half but the elevation 
errors were much smaller and the total time off target 
was much less than with the other two patterns. 

The errors in the first study were so large for in- 
experienced personnel that the Foxboro Company 
set up a laboratory experiment to determine the 
feasibility of training a man to operate simultane- 
ously the triple controls for simultaneously following 
a target in azimuth, elevation, and range. (221) Han- 
dle bar controls were used for tracking. Two devices 
were compared for the foot ranging— the standard 
opposed action pedal mechanism for the two feet and 
also a horizontal lever vertically pivoted in the center 
to be pressed by the two feet. Only the hexagon dot 
pattern with the central dot was used. 

The results with both the cross-bar and opposed- 
pedal forms of foot control demonstrate that under 
favorable conditions in two or three hours consider- 


able prohciency is possible in simultaneous hand and 
foot functions. All operators participating were suc- 
cessful in developing this coordination. However, 
the simultaneous functioning reduces the accuracy 
of separate manual and pedal functioning. Even 
under favorable conditions, 10 hours of practice 
amounting to 100 runs may be required to compen- 
sate the added difficulty of the triple performance. 
A training schedule seems important in this respect 
as evidenced by the result of three groups trained 
differently — simultaneous tracking and ranging, 
tracking first and ranging added, and ranging prac- 
tice first and tracking added. More accurate simul- 
taneous functioning followed 4 hours preliminary 
practice in a single function than 4 hours in the 
simultaneous functions. Under such conditions, 5 or 
6 hours of the latter were sufficient to compensate 
for the added difficulty. 

During the simultaneous performance, greater 
improvement was in the ranging when the initial 
training was only in ranging; it was in the tracking 
when the initial training was only in tracking. A 
significantly greater advantage was secured when the 
operators were first trained to track alone before 
ranging was added, presumably because of the sig- 
nificantly greater difficulty of the tracking operation. 
The ranging scores with the cross-bar and with the 
opposed-pedal controls did not differ in accuracy. 
The pedals may be rated slightly higher on three 
counts; less foot slippage, slightly better score, and 
unanimous operator preference. Foot controls of the 
sort investigated are open to criticism because the 
coordinations might break down under strain or 
distraction and because the feet are often needed to 
support or brace the body during manipulations of 
hand controls. 


RESTRIC I Kl) 


Chapter 17 

NEW INSTRUMENTS 


A s A RESULT of development during this period and 
XX as a result of some of the experiments discussed 
above, new instruments have been or are being de- 
veloped in which it is hoped to eliminate or at least 
better control some of the sources of error found in 
the present standard Service range finders. Part of 
this development has been the production of better 
and thermally more stable instrument parts already 
referred to in Section 5.4 above in this report. 

17 1 USE OF RHODIUM COATING 

Additional suggestions for the development of 
better instrument parts are found in two reports by 
the British Admiralty Scientific Research Depart- 
ment. The first of these describes the methods of 
manufacture employed by Adam Hilger, Ltd. for 
the production of plane parallel glasses to be used 
as mirrors in range finders. (59) It also discusses the 
making of rhodiumized windows for use in setting 
the infinity mark of range finders on the sun. Rho- 
dium is used for this purpose since it does not tarnish 
or corrode in sea air, although its reflecting power 
is considerably less than that of freshly deposited 
aluminum. The official British specifications are 
given. The second report discusses the special diffi- 
culties encountered in the rhodiumizing routine and 
shows how these difficulties have been overcome. (60) 
In recent years, the use of rhodium has gained great 
importance as a material for making surface reflect- 
ors and semi-transparent filters. Four properties of 
the rhodium films render this metal particularly 
suitable for these purposes: (1) high reflectivity, (2) 
neutral color in transmission, (3) hardness, and (4) 
resistance to chemical attacks. The report closes with 
a detailed description of the evaporation method of 
depositing this metal on glass surfaces. Many pre- 
cautions in the use of this technique are given. 

17 2 BASIC REQUIREMENTS OF 
RANGE FINDERS 

Several theoretical studies regarding the basic re- 
quirements for satisfactory range finders and range 
finder operation have been reported by the Prince- 


ton Branch of the Frankford Arsenal. The first of 
these discusses the basic physical and physiological 
requirements of a satisfactory instrument. (231) 
Because they crystallize the picture of range finder 
adequacy, it seems worth while to give the suggested 
requirements in detail, although some readers will 
not agree with all of the items as stated. 

1. The range finder shall have a standard path 
not subject to appreciable error. 

2. The working path of the range finder shall be 
the same as the standard path, shall be continuously 
compared with the standard path, or shall not be 
subject to appreciable change during one hour. 

3. Using a target at fixed range, temperature 
changes and other service conditions shall not cause 
more than 0.1 diopter of unbalanced focus difference 
between target image and reticle image. 

4. The difference in focus of target image and 
reticle image at a target range of 10,000 yards, a long 
tactical range, shall be less than 0.05 diopters. 

5. The spherical aberration of each image shall 
be less than 0.05 diopters. 

6. It shall be possible to adjust the interocular 
distance (the distance between the centers of exit 
pupils) with an accuracy of 0.1 millimeter and the 
setting shall maintain this accuracy for 1 hour. 

7. Exit pupils of 1 and at least 4 millimeters shall 
be provided. 

8. The product of the base length and the magni- 
fication shall be at least 324 feet. 

9. The loss of light in the range finder shall be not 
more than 35 per cent. 

10. Throughout the optical system, the glass sur- 
faces shall be coated to decrease reflection. 

11. The errors involved in data transmission shall 
correspond to triangulation errors of less than 0.5 
seconds of arc at all slant ranges greater than 2,000 
yards and to errors corresponding to slant range 
errors of less than 2 yards at shorter ranges. 

12. The angle between the directions of the optic 
axes of the oculars shall be less than 10 minutes. 

13. The reticles shall have been shown to be at 
least as good as current Service patterns by a satis- 
factory testing procedure. 

14. It shall be possible to make a height adjust- 



175 


176 


NEW INSTRUMENTS 


ment to within 10 seconds of the true adjustment. 

15. The range finder shall be provided with filters 
which have been shown by adequate tests to give as 
good range finder performance as those now used by 
the Services. 

16, The required rate of range-knob rotation, 
averaged over tactical situations weighed according 
to their importance, shall not be more than four times 
as large as the least average obtainable by a combina- 
tion of range-height conversion mechanisms and re- 
generative range. The reasons for the choice of these 
specifications are discussed in the text of the report. 

The second report by the Princeton Branch of the 
Frankford Arsenal report discusses a program of 
basic experiments of importance on range finder re- 
design. (283) The emphasis is upon physiological 
experiments on the potential observer and on data 
transmission to the director rather than upon experi- 
ments entirely on instrumentation of the range finder 
itself. Information is desired on such problems as 
the acuity of a normal or of slightly annetropic ob- 
servers, reticle design, and effect of increased power 
on acuity under poor seeing conditions as well as on 
data transmission problems. 

The Frankford Arsenal Princeton Branch reports 
a proposed program of study and design of a unified 
field modification of the Ml Height Finder. (247) 
The report includes a discussion of the principal 
problems which, as far as is known to date, must be 
solved in order that the performance of this instru- 
ment may be suitably improved. It also discusses in 
considerable detail the various devices, modifica- 
tions, and attachments which were then (1943) under 
consideration for design, construction, and testing 
in an effort to solve these problems in a practical 
manner. Another report from the same group out- 
lines suggested modifications to be made in a special 
Ml instrument for experimental purposes. (237) It 
is suggested that, in order to control perspective 
error, focusable objectives, special reticles, modified 
interpupillary scale and adjustment, diopter scales, 
controlled apertures, and height adjustment be intro- 
duced. To control stratification, modifications are 
suggested for the temperature tubes, introduction of 
Pyrex end reflectors of new design, internal-adjuster 
collimating reticles, and also changed penta-prism 
mounts are to be introduced to control problems of 
mechanical deformation. Other problems such as 
differential aberration, fungus growth, modified 
range drum, and coated optics are briefly discussed. 


In a final report on this topic by the Princeton 
Branch of the Frankford Arsenal, the results are of 
the integrated program of modification. (258) A long 
list of modifications already standardized, some de- 
signed to reduce or eliminate errors or mechanical 
difficulties in operation are given and discussed. 

17 3 SUGGESTED IMPROVEMENTS 

Until now in this section of the present report, 
there has been discussed modifications or changes in 
the existing range finders. Below will be given a 
discussion of instruments which involve new ap- 
proaches to the problem of obtaining range, or novel 
approaches to the elimination or control of some of 
the more important sources of error. 

A captured German 4-meter range finder. Model 
Em 4 m R 40, was studied by the Fire Control Design 
Division of the Frankford Arsenal. (228) This is a 
stereoscopic instrument capable of reading ranges 
from 1,200 to 100,000 meters. Ortho-stereoscopic and 
pseudo-stereoscopic fields can be selected at either 
20 or 32 power. It has coated optics throughout. The 
range finder proper, the outside of which is a one- 
piece aluminum alloy casting except for end boxes, 
bearings, and auxiliary plates, rests on a cradle con- 
taining elevation and azimuth hand wheels and 
mechanisms. This in turn rests upon a tripod, which 
is leveled by two horizontal screws, 90 degrees apart. 

Little insulation was found on the range finder 
proper, although metal sunshades extend on both 
sides from the main bearing housings to the end 
boxes. The interior of the end boxes is insulated by 
sheets of some aluminum alloy. The instrument has 
no inner tube, and has an optical bar only for reticle 
collimation. Fixtures for optical elements and in- 
ternal mechanisms are positioned on the inside of 
the casting by machining a surface where attachment 
is desired and then hand-lapping the fixture into 
place. 

Because the elevation scale goes from 0 to 90 de- 
grees, it is believed that the instrument is an anti- 
aircraft range finder, although the only range-to- 
height device is a conversion table on the right end 
box. It is thought that the range finder proper can 
also be placed on an antiaircraft director, into which 
it would then feed slant range, by means of a clutch 
device which is connected directly to the range knob 
mechanism. The instrument was made by Zeiss. The 
controls, main optical path, optical path of reticles. 


D 


RESTRICTED 


SUGGESTED IMPROVEMENTS 


177 


daylight and artificial illumination of reticles is de- 
scribed. A collimator, as a separate instrument, was 
captured with this range finder. 

The same instrument and the collimator are also 
described by the Aberdeen Proving Ground. (45) 
Brief descriptions of operational procedures, photo- 
graphs and schematics of the range finder and its 
collimator are given. The important new aspect of 
this instrument is the elimination of the optical bar 
in its construction and the attachment of the optical 
parts directly to the main tube. The reticles of this 
instrument are new and have been discussed above 
in the section on reticle design. The comparative 
tests of this instrument by the Aberdeen Proving 
Ground (46) are also discussed above in the section 
on comparison of instrument types. 

An instrument of ortho-pseudo stereoscopic type 
was developed by the Eastman Kodak Company and 
is described in detail in a report published by them. 
(204) This instrumental design eliminates reticles 
and hence elminates all parallax errors provided the 
right and left hand telescopic systems are accurately 
matched. The field of this instrument shows two 
images of the target in the upper and lower halves 
of the field. These may be varied in stereoscopic 
distance and the problem of the operator is to bring 
the two images into the same stereoscopic plane. A 
limitation of this present design is that a standard 
Ml Height Finder was modified for this purpose 
“with the fewest possible changes” which decidedly 
limited the development possibilities for the de- 
signers. It was hoped that this instrument would give 
better results under poor seeing conditions. In this 
case the ortho-pseudo field would present two images 
of similar brightness and background contrast for 
comparison instead, as in the case of reticle type 
stereoscopic instruments, of target and reticle images 
of different brightness and different background 
contrasts. An ortho-pseudo instrument presents the 
difficulty in adjusting the two stereoscopic images to 
allow for increasing size as range decreased and in 
maintaining, by exceptionally good tracking, the two 
images near the halving line. 

Results of comparative tests of the Eastman ortho- 
pseudo range finder and standard Army height 
finders have already been described above and ap- 
pear in a report from the Princeton Laboratory at 
Fort Monroe. (351) These two reports are attached 
as supporting data to a Report to the Services issued 
by the Fire Control Division of NDRC. (21) This 


report summarizes these results as follows. When all 
the available information is taken into account, there 
appears to be good reason to believe that a well 
designed ortho-pseudoscopic range finder in the 
hands of a well-trained crew would be more accurate, 
and more consistently accurate, than any other in- 
strument so far devised. A similar amount of turn 
of the range knob should give twice the amount of 
movement between images in the ortho-pseudo situ- 
ation as in the reticle type of instrument. The report 
further states that, in time of peace, or if procure- 
ment of optical range finders were not so difficult, an 
energetic prosecution of this development would 
obviously be warranted. 

The Princeton Branch of the Frankford Arsenal 
suggests the design of optical range finders employ- 
ing polarizing beam splitting surfaces to combine all 
of the target beams to be presented to an observer. 
(248) In this report, a number of schemes for arrang- 
ing the polarizing surfaces to combine target beams 
in the desired fashion are presented together with 
various methods for obtaining the common types of 
field presentation — reticle stereoscopic, erect split 
field coincidence, and invert ortho-pseudo. A discus- 
sion of the general optical characteristics is given 
and the conclusion is drawn from this discussion 
that the special features possessed by this type of 
range finder are such that it would be a good, versa- 
tile instrument which would be relatively simple to 
construct and maintain. It is believed that the chief 
application of this principle would be as a short- 
base high-power instrument under conditions where 
two observers sharing the same range standard and 
using different viewing instruments can be utilized 
to advantage. At the time of the publication of this 
report (February 1944) there appeared to be con- 
siderable difference of opinion as to whether the art 
of polarizing beam splitting had reached a degree 
of excellence sufficient for this application. Another 
report from the same source considers the applica- 
tion of this polarizing beam splitting type of range 
finder for tank use and considers the advantages and 
disadvantages attendant upon such application. 
(252) 

In this connection, the Eastman Kodak Company 
subsequently published a description of work on a 
polarizing beam-splitter by evaporating alternate 
films of high and low index materials on glass. (214) 
The report discusses the principles of such a device 
and the methods and difficulties of manufacture. 


RESTRICT 


ictEit^ 


178 


NEW INSTRUMENTS 


Although these experiments have demonstrated that 
it is possible to make a polarizing beam splitter with 
characteristics almost the same as those predicted by 
theory, the Eastman Kodak group point out three 
primary limitations to this type of prism. (1) Al- 
though the transmitted beam is efficiently polarized 
over a wide angle, the reffected beam is limited to a 
narrow angle of about 2 to 3 degrees. (2) The prism 
is effective for a limited band of wave lengths, slightly 
less than the width of the visible spectrum. (3) Very 
careful control is necessary to make satisfactory beam 
splitters inasmuch as the problems encountered in 
the vacuum evaporation of multiple films are, in 
general, still formidable. 

The Eastman Kodak Company designed and pro- 
duced almost complete layout drawings for a 13i/2 
foot superimposed type range finder. (212) It was 
suggested that, with the increased use of radar range 
finding devices, the optical range finder might be- 
come merely a standby instrument. Hence a range 
finder of the superimposed type would be simpler 
in operation and less expensive to manufacture than 
the stereoscopic instrument. The designed instru- 
ment was to have 24 x magnification and a field angle 
of li /2 degrees and was calculated to be effective to 
20,000 yards. The instrument is described in the 
report. After approximately 90 per cent of the de- 
tailed drawings had been completed, the project 
was discontinued. 

As a result of the foregoing experiments and the 
resultant better appreciation of the sources of error 
in present Service range finders, a meeting was called 
at the Frankford Arsenal to discuss the feasibility of 
completely new range finder design on March 15, 
1943 under the auspices of NDRC. There were pres- 
ent Army representatives from Ordnance and the 
Antiaircraft Artillery Command, from the Naval 
Bureau of Ordnance, and from each of the three 
principal manufacturers— Bausch and Lomb Optical 
Company, Eastman Kodak Company, and Keuffel 
and Esser. The result of this meeting was a second 


meeting on April 16, 1943 at the University Club in 
Boston for the discussion of problems. A third meet- 
ing was held in Washington, D. C. on September 15, 
1943 by which time a Steering Committee for the 
“Super Range Finder” had been organized including 
Army and Navy, and NDRC personnel and repre- 
sentatives of the three manufacturers. At this time, 
the manufacturers presented schemata of types of 
range finders which they would like to develop. The 
Steering Committee authorized the design of two 
different types of instrument by Bausch and Lomb, 
two by the Eastman Kodak Company and one by 
Keuffel and Esser. In addition the National Bureau 
of Standards was working on a new design entirely 
apart from the NDRC development. Three of these 
designs were selected as most promising and Bausch 
and Lomb was requested to design and build two 
of them and Eastman Kodak Company one. Two 
instruments of each of these three designs were 
authorized so that the Army and Navy could run 
separate simultaneous tests. Various details of de- 
sign and construction were decided upon. Still an- 
other meeting of this group was held in Rochester, 
New York on July 12 and 13, 1944. At this time 
progress reports were made by each of the three 
manufacturers. There was also a demonstration of 
two “breadboard models” developed by the Eastman 
Kodak Company. Detailed reports on other matters 
were presented at this time. 

Schemata of these designs are presented in the 
conference reports referred to above. No details are 
available at the date of the present report since none 
of these instruments is as yet entirely completed 
(October, 1944). Schemata of the designs will be 
found, for Bausch and Lomb (112, 113) and, for 
Eastman Kodak Company (197, 200, and 201). 

A more detailed description of two forms of the 
Eastman Kodak Company designs are to be found 
in a subsequent report. (213) This report also sum- 
marizes experiments and developments on compon- 
ent parts of the super range finder. 


RESTRICTE 


BIBLIOGRAPHY 


Numbers in parentheses following each entry indicate the chapter(s) of this volume in which the 
entry is discussed. Numbers such as Div. 7-210.1 1-M2 indicate that the document listed has been 
microfilmed and that its title appears in the microfilm index printed in a separate volume. For 
access to the index volume and to the microfilm, consult the Army or Navy agency listed on the 
reverse of the half-title page. 


REPORTS TO THE SERVICES [RS] ISSUED BY 
SECTION D2 AND DIVISION 7 

1. Reduction of Range and Height Finder Errors Due to 

Temperature Stratificatiori, OSRD 49, RS 12, Aug. 29, 
1941 (5). Div. 7-210.1 1-M2 

2. Tests of Eynotional Stability, OSRD 344, RS 18, Brown 

University, Jan. 5, 1942, p. 2 (13). Div. 7-220. 11-M4 


17. The Eastman Trainer for Range Finder Observer, OSRD 
989, RS 39, NDCrc-186, OEMsr-56, Eastman Kodak Co., 
Princeton University, Oct. 17, 1942, p. 3 (14). 

Div. 7-220.213-M5 

18. Perspective Effects in Stereoscopic Range Finders, OSRD 
1128, RS 40, NDCrc-186, Princeton University, National 
Bureau of Standards, Dec. 30, 1942, p. 15 (4) (5) (13). 

Div. 7-210.15-M2 


3. Effect of Stopping Doiun End IVindoivs of Range Finders, 
OSRD 378, RS 21, Feb. 11, 1942, p. 2 (4). Div. 7-210-Ml 

4. Selection of Stereoscopic Range Finder Operators, OSRD 

410, RS 22, Feb. 23, 1942, p. 5 (13). Div. 7-220.14-M12 

5. The Precision, Consistency, and Accuracy of Visual Range 

Observations, Ross A. McFarland, Alfred H. Holway, and 
others, OSRD 578, RS 25, May 14, 1942, p. 2 (2). 

Div. 7-220.12-M6 

6. The Development of Short Base Range Finders, OSRD 
591, RS 26, May 15, 1942, p. 6 (15). Div. 7-210.17-Ml 

7. The Relation of National Research Council Neurotic In- 

ventory Scores and Intelligence, Clarence H. Graham, 
OSRD 618, RS 27, Brown University, May 1942, p. 2 (13). 

Div. 7-220.1 1-M8 

8. Relative Position of Reticle and Target in the Stereo- 

scopic Range Finder, OSRD 622, RS 28, May 27, 1942, 
p. 11 (10). Div. 7-210.1 1-M4 

9. Relation of Accuracy and Variability of Stereoscopic Anti- 

aircraft Range Finding Operation to Correct Interocular 
Setting of the Instrument, OSRD 626, RS 29, May 25, 1942, 
p. 3 (4). Div. 7-210.11 -M3 

10. Description of Instruments for Use in the Selection of 

Stereoscopic Range Finder Operators, Merrill M. Flood, 
OSRD 592, RS 30, Princeton University, May 18, 1942, 
p. 2 (13). Div. 7-220.14-M3 

11. Selection of Stereoscopic Range Finder Operators, Merrill 

M. Flood, OSRD 721, RS 31, NDCrc-186, Princeton Uni- 
versity, July 7, 1942, p. 2 (10). Div. 7-220.14-M9 

12. Analysis of Factors Effective in the Use of the Stereoscopic 

Range and Height Finder, OSRD 862, RS 32, Sept. 9, 1942, 
p. 2, 20 (9). Div.7-220.213-M3 

13. The Chromatic Dispersion of the Human Eye and its 
Possible Influence on Stereoscopic Range Finding, G. A. 
Fry, Charles G. Bridgman, and others, OSRD 923, RS 35, 
OEMsr-637, Ohio State University, Oct. 1, 1942, p. 2 (11). 

Div.7-230.1-Ml 

14. The Influence of Atmospheric Scattering in Range Find- 

ing, G. A. Fry and Charles S. Bridgman, OSRD 930, 
RS 36, OEMsr 637, Ohio State University, Oct. 12, 1942, 
p. 3 (11). Div. 7-210.1 11-M2 

15. The Effects of Loud Sounds on the Accuracy of Azimuth 

Tracking and of Stereoscopic Range Finding, OSRD 1001, 
RS 37, Nov. 27, 1942, p. 3 (9). Div. 7-220.18-M4 

16. Temperature Effects in Stereoscopic Height Finders, 

OSRD 994, RS 38, NDCrc-186, OEMsr-56, Princeton Uni- 
versity, Eastman Kodak Co., National Bureau of Standards, 
Oct. 8, 1942, p. 8 (5). Div. 7-210.21 -M3 


19. Comparison of Precision at 12 ayid 24-Poiver of Observa- 

tions Taken icith the Ml Heightfinder for 6 Ground 
Targets and Moving Aerial Targets, OSRD 1180, RS 41, 
Princeton University, Jan. 20, 1943, p. 3 (6). 

Div.7-210.15-Ml 

20. Comparative Test of Coincidence and Stereoscopic Height 

Finders, Merrill M. Flood, OSRD 1129, RS 42, NDCrc-f86, 
Princeton University, Oct. 28, 1942, p. 3 (3) 

Div.7-210.14-Ml 

21. Ortho-pseudoscopic Range Finder, OSRD 995, RS 43, 

NDCrc-186, OEMsr-55, Eastman Kodak Co., Princeton 
University, Oct. 29, 1942, p. 3 (3) (17). Div. 7-210.16-Ml 

22. Stereo and Vernier Acuity as Affected by Metrazol, Loss 
of Sleep, Benzedrine, Variations in Blood Sugar, and Hy- 
perventilation, OSRD 1007, RS 44, Nov. 28, 1942, p. 16 (9). 

Div. 7-220. 12-M 18 

23. Field Tests of Eastman Kodak 15" and Polaroid 43" Base 
Range Finders, OSRD 1112, RS 45, Dec. 1942, p. 20 (15). 

Div.7-210.17-M3 

24. Learning Rate and Training Program for Stereoscopic 

Range Finder Operators, Merrill M. Flood, OSRD 1203, 
RS 48, NDCrc-186, Princeton University, Feb. 15, 1943, 
p. 2 (14). Div. 7-220.14-M10 

25. Manual for Selection of Stereoscopic Range Finder Oper- 

ators, Merrill M. Flood, OSRD 1210, RS 49, NDCrc-186, 
Princeton University, Feb. 17, 1943, p. 2 (13). 

Div. 7-220.14-Mll 

26. Retention of Helium in Range and Height Finders, OSRD 

1258, RS 50, NDCrc-186, OEMsr-562, Princeton University, 
American Gas Association Testing Laboratories, Mar. 1, 
1943, p. 6 (5) (10). Div.7-210-M2 

27. Test Made of Range Finder Optical Systems at California 

Institute of Technology , J. A. Anderson, Ira S. Bowen, and 
others, OSRD 1257, RS 51, California Institute of Tech- 
nology, Feb. 20, 1943, p. 3 (2). Div.7-210.13-M4 

28. Sex Differences in Stereoscopic Ranging and in Visual 

Tracking, OSRD 1250, RS 52, Feb. 23, 1943, p. 14 (9). 

Div. 7-220.34-M2 

29. The Effects of Motivation and of Fatigue on Stereoscopic 

Ranging and Direct Tracking, OSRD 1478, RS 56, Tufts 
College, 1943, p. 2 (9). Div. 7-220.16-M12 

30. Descriptive Summary of Errors Made with the M-1 Height- 

finder During Field Tests at Fort Monroe, OSRD 1363, 
RS 54, Princeton University, Apr. 15, 1943, p. 2 (14). 

Div.7-210.21-M4 

31. Comparative Study of Internal Adjuster Targets of Range 

and Height Finders, OSRD 2061, RS 69, OEMsr-570, 
Brown University, Nov. 29, 1943, p. 5 (7) (16). 

Div. 7-210.1 12-M3 



179 


180 


BIBLIOGRAPHY 


52. Factors Influencmg the Use and Construction of Range 
and Height Finders, OSRD 3162, RS 70, Jan. 5, 1944, 
p. 4 (11). Div.7-210-M4 

33. A Study of Factors Determining Accuracy of Tracking by 

Means of Hand Wheel Control, OSRD 3451, RS 71, 
OEMsr-453, The Foxboro Co., Sept. 30, 1942, p. 40 (10). 

Div. 7-220.31 1-Ml 

34. A Supplemental Study of Factors Determining Accuracy 

of Tracking by Means of Handivheel Control, OSRD 3452, 
RS 72, OElsisr-453, The Foxboro Co., Dec. 7, 1942, 
p. 26 (10). Div. 7-220.3 11 -M2 

35. Handivheel Speed and Accuracy of Tracking, OSRD 3453, 

RS 73, OEMsr-453, The Foxboro Co., May 12, 1943. 
p. 17 (10). Div. 7-220.311 -M3 

36. Inertia, Friction, and Diameter in Handwheel Tracking 

(Report 4), OSRD 3454, RS 74, OEMsr-453, The Foxboro 
Co., Sept. 15, 1943, p. 20 (10). Div. 7-220.31-Ml 

37. Relative Accuracy of Handwheel Tracking With One 

and Both Hands (Report 5), OSRD 3455, RS 75, OElSIsr- 
453, The Foxboro Co., Nov. 20, 1943, p. 14 (10). 

Div. 7-220.3 11-M4 

38. Interocular Adjustment of Stereoscopic Range Finders, 

OSRD 3200, RS 76, Jan. 13, 1943, p. 7 (4). 

Div. 7-210.1 12-Ml 

39. Tank Fire Control. Visual Range Estimation. Range Find- 

ers and Range Finder Fields, OSRD 3259, RS 77, Contract 
Symbol 2488, Bausch and Lomb Optical Co., Feb. 1944, 
p. 10 (15). Div.7-210.1-M2 

40. The Effect of Imperfections in the Reticle Field of Ster- 

eoscopic Rangefinders, Roberta M. Daley, Clarence H. 
Graham, and others, OSRD 3866, RS 83, OEMsr-1059, 
Brown University, July 1944, p. 3 (16). Div. 7-210.32-M3 

41. Reduction of Temperature Effects in Pentareflectors and 

Optical Bars, OSRD 4527, RS 85, OEMsr-952, OEMsr-1016, 
Bausch and Lomb Optical Co., Eastman Kodak Co., Oct. 
11, 1944, p. 4 (5). Div.7-210.12-M3 

42. Studies of Direct Tracking and Simultaneous Stadiametric 
Ranging, OSRD 4876, RS 87, Dec. 1944, p. 3 (15). 

Div. 7-220.33-M4 

43. Comparative Studies of Slewing and Central Tracking 
with a Variety of Types of Machine Gun Sights, OSRD 
4877, RS 88, OEMsr-453, The Foxboro Co., Jan. 1945, p. 4. 

Div.7-220.32-Ml 

44. Studies of the Design of Opaque Reticles for Stereoscopic 
Range and Height Finders, C. H. Graham, Lorrin A. 
Riggs, and others, OSRD 4925, RS 89, OEMsr-1059, Brown 
University, March 1945, p. 4 (16). Div. 7-210.31-M9 

ABERDEEN PROVING GROUND 

45. Heightfinder M36, 4 -Meter Base, German, FMCF-99; and 

Collimator for Heightfinder, 4-Meter Base, German, 
FMCF-IGO, Dec. 9, 1943, p. 28 (17). 

46. Comparative Tests of the Standard U.S. Heightfinder Ml 

and the German Rangefinder Em 4 m R40, Sept. 21, 1944, 
p. 10 (3). 

47. Comparison Between German M40, 4-Meter Base Range 

Finder and American Ml Height Finder, Nov. 14, 1944, 
p. 14 (3) (17). 

ADMIRALTY RESEARCH LABORATORY 

48. Examination and Test of Eastman Kodak, 15 -inch Base, 

Superimposed Field Rangefinder, Dec. 31, 1943, p. 4 (15). 


49. Test of 43" Base Stereoscopic Rangefinder, manufactured 

by Polaroid Corporation, Ltd, Aug. 12, 1943, p. 3 (15). 

50. Ml Heightfinder. The Removal of Elevation Errors by 

Circumferential Air Stirring, Aug. 3, 1943 (5). 

51. The Effect on Range Observations of Unequal Trans- 

mission of the Two Telescopic Systems of a Stereoscopic 
Rangefinder, Apr. 7, 1944, p. 5 (8). 

52. Trial on Fixed Scale, 7(4 Meter Base, Levallois Stereo- 

scopic Rangefinder, Employing Observers Who Had Had 
No Previous Stereoscopic Training, Dec. 1, 1942, p. 8 (15). 

53. Preliminary Trial to Determine the Suitability of the No. 

12 Infantry Rangefinder for Tank Use; Conducted at 
Cobharn, June 9, 1943, p. 19 (15). 

54. Report on a Trial to Estimate the Value of a Rangefinder 

in Tank Gunnery, Aug. 20, 1943, p. 28 (15). (Included 

in Ref. 39.) 

55. The Elevation Errors of Heightfinders Types U.D. 4 and 

U. K. 4, Sept. 9, 1940, p. 32 (5). 

56. The Elevation Errors of Height and Rangefinder No. 10 

Type F. Q. 25, Sept. 9, 1940, p. 32 (5). 

57. The Elevation Errors of the Goerz Heightfinder No. 8, 

Jan. 29, 1941, p. 14 (5). 

58. The Elevation Errors of Army Heightfinder No. 3. Type 

U.B. 7, Feb. 20, 1941, p. 15 (5). 

59. Making Plane and Plane Parallel Mirrors and Wi)idoivs 

for Rangefinders, Including Rhodiumised Windows for 
Setting the Infinity Mark of Range-Finders on the Sun, 
F. Twyrnan, 1944, p. 9 (17). 

60. The Production of Good Rhodium Films on Glass by the 

Evaporatioji Method, . Zehden, June 30, 1944, p. 14 (17). 

61. Rangefinder Performance Computer, July 1944, p. 3 (14). 

AMERICAN GAS ASSOCIATION 

62. Performance of Helium Retention Apparatus, Jan. 7, 1943, 

p. 36 (5). (Included in Ref. 26.) 

63. Report on the Helium Retention Apparatus for Use on 

U. S. Navy Range Finders, Jan. 8, 1943, p. 17 (5). 

(Included in Ref. 26.) 

64. Effect of Pressure Compensating Bellows and Automatic 

Replenishment System on Maintenance of Uniform Pres- 
sure and Helium Purity in Height Finder, Jan. 21, 1943, 
p. 5 (5). (Included in Ref. 26.) 

65. Design and Development of Helium Retention Apparatus 
for use on Height Finders, Feb. 12, 1943, p. 25 (5). 
(Included in Ref. 26.) 

APPLIED MATHEMATICS PANEL 

66. The Dependence of the Precision of a Stereoscopic Range- 

finder Upon the Magnification Employed. Sept. 13, 1943, 
p. 13 (6) (13). 

67. The Dependence of the Precision of a Stereoscopic Range- 
finder Upon Base Length, Jan. 21, 1944. p. 24 (6). 

APPLIED PSYCHOLOGY PANEL (NRC) 

68. Training Manual for Stereoscopic Height Finder Ob- 

servers, OSRD 1544, June 15, 1943, Chaps. 1-5, pp. 1-66 
(9) (14). APP-215-M1 

69. A Study of Backlash Betiveen the Main Bearing Race and 
the Bevel Pinion on Ml and M2 Height Finders, Project 
Report 8, OEMsr-815. Sept. 9, 1943, p. 6 (8). APP-652-M4 

also Div. 7-210.2-M3 


I^strTcteiT 


BIBLIOGRAPHY 


181 


70. Training Manual for Stereoscopic Height Finder Ob- 

sey-vers, OSRD 1730, Aug. 12, 1943, Chaps. 6-19, pp. 67-221 
(9) (14). APl’-215-Ml 

71. Training Manual for Stereoscopic Height Finder Observ- 

ers, OSRD 2008, Oct. 26, 1943, Chaps. 20-37, pp. XVII, 
223-369 (9) (14). APP-215-M1 

72. Results Obtained from Testing Recruits with the New 

London NRC Questionnaire at the Neivport Naiml Train- 
ing Station, OSRD 3040, Dec. 6, 1943, p. 4 (13). 

APP-324-M1 

73. The Selection of Stereoscopic Height Finder Observers, 

OSRD 1790, Aug. 14, 1943, p. 31 (13) (14). APP-120-M1 

74. Height of Image Adjustment on the Stereoscopic Height 

Finder, OSRD 1967, Sept. 15, 1943, p. 8 (12). APP-652-M5 

75. A Comparison of Methods for Setting the Oculars of the 

Height Finder at the Interpupillary Distance of the 
Observer, OSRD 1968. Sept. 15, 1943, p. 6 (4). 

APP-653-M2 

76. The Relationship of Visual Acuity to Acuity of Stereo- 
scopic Vision, OSRD 2087, Sept. 15, 1943, p. 15 (13). 

APP-121-M2 

77. An Instrument for Tracking Training, OSRD 1497, May 

4, 1943, p. 6 (14). ' APP-611.2-M1 

78. The Precision of Setting the Oculars of the Rangefinder 

with I liter pupillary Distance Templates of Different De- 
sign, OSRD 3273, Feb. 2, 1944, p. 10 (4). APP-653-M3 

79. The Calibration of Army Height Finders, OSRD 3375, 

Mar. 18, 1944, p. 7 (7). ' APP-652-M6 

80. The Precision of Internal Adjustment Settings of the 

Mark 42 Rangefinder, OSRD 3618. Apr. 24, i944, p. 8 
(7). APP-651.1-M1 

81. The Design, Accuracy, Construction and Use of a Range 

Correction Computer, OSRD 3509, Mar. 22, 1944, p. 11 
(14). APP-640-M1 

82. The Reliability and Precision of the NDRC and Bausch 

and Lomb Interpupillometers, OSRD 3475, Mar. 29, 1944, 
p. 21 (13). APP-112.2-M1 

83. The Relationship of Errors in Height and Slant Range 

Readings Made by Stereoscopic Observers, OSRD 3594, 
Apr. 29, 1944, p. 5 (14). APP-216-M1 

84. Inter-relationships Among Seven Tests of Stereoscopic 

Acuity and the Relationship betiveen Two Tests of Visual 
Acuity and Two Tests of Phorias, Project Memorandum 
12, Mar. 24, 1944, p. 26 (13). APP-121-M3 

85. The Comparability of Formats A & B of the Personal 

Inventory, OSRD 3582. Apr. 21, 1944, p. 5 (13). 

APP-321-M4 

86. The Design, Accuracy, Construction and Use of a Range- 
finder Slide Rule, OSRD 3678, May 1, 1944, p. 14 (14). 

APP-651-M1 

87. Description of the Tufts Tracking Trainer, OSRD 3286, 

Feb. 5, 1944 (10). APP-611.2-M4 

88. A Study of the Tufts Tracking Trainer as a Selection and 

Training Device for Trackers on the Director, M7, OSRD 
3606, Apr. 24, 1944, p. 11 (10). APP-611.2-M7 

89. The Personal Iiwenlory. Short Form (Format C); Psychi- 

atric J’alidation on a Pre-Test Basis, OSRD 3604, May 1, 
1944, p. 13 (13). APP-321-M5 

.00. Final Summary of Work on the Selection and Training 
of Stereoscopic Height Finder Observers, June 14, 1944, 
OSRD 3773, p. 22 (13) (14). APP-120-M2 


91. The Design, Construction, and Use of a Motor Drive Pro- 

viding Range Movement and Tracking Errors for the 
Mk 2 and M2 Stereoscopic Trainers, OSRD 3907, July 17, 
1944, p. 9 (14). APP-651-M2 

92. A Comparison of Personal Inventory Scores with Service 

Records One Year After Testing, OSRD 3755, June 10, 
1944, p. 15 (13). / APP-321-M6 

93. An Investigation of the Effect of Brightness Contrast 

upon Operators’ Errors in Ranging on Aerial Targets 
with the Mark 42 Rangefinder, OSRD 4380, Nov. 27, 
1944, p. 14 (11). APP-651.1-M3 

94. Results from the Long and Short Forms of the Personal 

Inventory and the General Classification Test, OSRD 
3962, July 31, 1944, p. 10 (13). APP-321-M7 

95. Final Report in Summary of Research on the Personal 

Inventory and Other Tests, OSRD 3963, Aug. 1, 1944, 
p. 33 (13). APP-321-M8 

96. The Influence of the Visual Tasks Required of Personnel 

in the 16 Weeks Fire Controlmen (0) Training Course 
upon their Visual Proficiency, OSRD 3970, Aug. 1, 1944, 
p. 26 (14). APP-216-M3 

97. The Precision of Internal Adjustment Settings by Student 
Operators and Experienced Operators on Stereoscopic 
Rangefinders, OSRD 3997, Aug. 10, 1944, p. 9 (7). 

APP-216-M4 

98. A Comparison of Five Methods of Calibrating the Mark 
42 Rangefinder, OSRD 3977, Aug. 4, 1944, p. 17 (7). 

APP-651.1-M2 

99. Manual for Use in the Selection of Fire Controlmen (0), 
Aug. 22, 1944, OSRD 4050, p. 92 (13). APP-lll-Ml 

100. Learning Curves for Operators of Stereoscopic Range- 
finders, OSRD 4349, Nov. 21, 1944, p. 22 (14). 

APP-216-M5 

101. A Folloiv-Up Study of the Efficiency of the Projection 

Eikonomcter Test in Predicting the Performance of 
Stereoscopic Height Finder Operators, OSRD 4352, Nov. 
21, 1944, p. 11 (13). APP-122-M4 

ARMAMENT RESEARCH LABORATORY 

102. A. Use of Helium in Ml Heightfinder, March 1942, p. 2 

(5). 

103. B. Examination of Elevation Errors of U.S. Stereoscopic 

Heightfinder, Type Ml, July 1, 1943, p. 33 (5). 

104. C. Tiuo Station Rangefinder, Dec. 9, 1941, p. 4 (17). 

105. D. Reduction of Range & Heightfinder Errors Due to 

Temperature Stratification, March 1943, p. 3 (5). 

106. E. Memorandum on the Construction and Calibration of 

the NDRC Interpupillometer, June 19, 1944, p. 10 

(13). 

BAUSCH & LOMB OPTICAL COMPANY 

107. Interocular Setting of Range Finders Having Persjjective 

Errors, Dec. 14, 1943, p. 11 (4) (13). (Included in 

Ref. 38.) 

108. Tests of Range Finders for Armored Force Use Con- 
ducted at Fort Knox, 3-20-43 to 3-14-43, June 4, 1943^ 
p. 27 (15). (Included in Ref. 39.) 

109. Tests of Range Finders for Armored Command Use Con- 

ducted at Fort Knox 6-14-43 to 6-24-43, July 30, 1943,. 
p. 32 (15). (Included in Ref. 39.) 


RESTRICTED 


182 


BIBLIOGRAPHY 


no. 

111 . 

112 . 

113. 

114. 

115. 

116. 


117. 

118. 

119. 

120 . 

121 . 

122 . 

123. 

124. 


125. 


126. 


127. 


‘ 128 . 


Range Finder and Range Estimation Tests at Fort Knox. 
Discussion, Sept. 10, 1943, p. 10 (15). (Included in Ref. 

39.) 

Test of Precision and Accuracy of Five Different Types 
of Rangefinders, Sept. 14, 1943, p. 7 (15). (Included in 

Ref. 39.) 

1. Auto-Collimating Stereoscopic Rangefinder with Mini- 
mum Stratification Influence (Proposal 1) (2 charts) 
O. H. Wolferts, OEMsr-1016, Sept. 10, 1943. 

Div.7-210.18-M3 

2. Same title— (Proposal 2) (2 charts) O. H. Wolferts, 

OEMsr-1016, Sept. 10, 1943. Div. 7-210.18-M3 

A Proposed Optical Design for a Super Range Finder of 
the Stereo Reticle Type, OEMsr-1016, 1943, p. 4 (17). 

Div. 7-210.31-M2 

Laboratory Tests on Comparative Performance of Invar 
and Steel Height Finder Optical Bars (Report 1), OEMsr- 
1016, June 25, 1944. p. 48 (5). Div. 7-210.22-Ml 

Superimposed Image Range Finder, 1943, p. 16 (2). 

Laboratory Tests 07i Comparative Performance of Invar 
and Steel Height Finder Optical Bars, Aug. 29, 1944, p. 
14 (5). (Included in Ref. 41.) 

BROWN UNIVERSITY 

Effect of Electric Shock Trials on Fusion (Progress Re- 
port of Project 10), OEMsr-66, Sept. 15, 1941, p. 18 (13). 

Div. 7-220.11 -Ml 

Electrical Shock and Apprehensioii Tests on Six British 
Seamen (Progress Report of Project 10), OEMsr-66, Nov. 
25, 1941, p. 20 (13). Div. 7-220.1 1-M3 

Progress Report: 1. Analysis of the Data Obtained at Fort 
Monroe. 2. Experiments on Reticle Desigti, Feb. 9, 1942, 

p. 60 (16). 

Psychological Tests of Brown University Students (Mem- 
orandum to Steering Committee of Project 10), Clarence 
H. Graham, OEMsr-66, Mar. 27, 1942. p. 3 (13). 

Div. 7-220.1 1-M5 

Further Analysis of the NRC Neurotic Inventory (Memo- 
randum to Steering Committee of Project 10), Clarence 
H. Graham, OEMsr-66, Apr. 9, 1942, p. 3 (13). 

Div. 7-220.1 1-M6 

Psychological Tests (Progress Report of Project 10), 
OEMsr-66, Apr. 25, 1942, p. 26 (13) Div. 7-220.1 1-M7 

Preliminary Analyses of the NRC Neurotic Inventory, 
OEMsr-66, June 15, 1942, p. 8 (13). Div. 7-220.1 1-M9 

Preliminary Results with a Battery of Tests Developed 
for the Selection of Emotionally Unstable Service Per- 
sonnel, OEMsr-66, June 15, 1942, p. 16 (13). 

Div. 7-220.1 1-MlO 

A Method for Determming Reaction Time of Binocular 
Fusion Under Conditions of Stress, OEMsr-66, June 15, 
1942, p. 14 (13). Div. 7-220.1 1-Mll 

An Apparatus for the Coinparison of Stereoscopic Settings 
with Different Reticles Together with Some Illustrative 
Results, OEMsr-1059, June 15, 1942, p. 21 (16). 

Div. 7-210.33-Ml 

Summary Report of Work Done Under Contract OEMsr- 
66. Project No. 10, Section D-2 NDRC, Clarence H. 
Graham, Lorrin A. Riggs, and others, OEMsr-66, July 20, 
1942, p. 5 (13). Div. 7-220.11-M12 

A Comparison of Wonderlic and Otis Intelligence Scores 
for Submarine Men and Students (Progress Report 1), 
Sept. 2, 1942, p. 6 (13). Div. 7-220.15-M7 


129. Shock-fusion Perfortnance of Submarine Men (Progress 

Report 2), OEMsr-66, Sept. 9, 1942, p. 8 (13). 

Div. 7-220.1 1-M13 

130. Evaluation of Procedures Used in the Broicn University 

Research at the U.S. Submarine Base, New London, 
Connecticut (Progress Report 3), OEMsr-570, Sept. 21, 
1942. p. 17 (13). Div. 7-220.1 1-M14 

The Broivn Stereoscopic Trainer (Progress Report 4), 
Sept. 23, 1942. p. 10 (14). Div. 7-220.213-M4 

The Two-Hand Coordination Test Performance of Sub- 
marine Men (Progress Report 5), OEMsr-66, Sept. 28. 

1942, p. 1 (13). Div. 7-220.1 1-M15 

133. The Effect of Loud Sounds on the Ability of an Observer 

to Maintain Continuous Stereoscopic Contact as in Range 
Finding (Progress Report 6), Oct. 14, 1942, p. 10 (9). 

Div. 7-220.18-M3 

134. Item Analysis No. IV on the NRC Neurotic Inventory 

Tests Taken by the Submarine School Population Prog- 
ress Report 7), OEMsr-66, Nov. 3, 1942, p. 6 (13). 

Div. 7-220.11-M16 

135. Selectivity of a Battery of Tests. Psychiatric Criterion 

(Progress Report 8), OEMsr-570, Dec. 3, 1942, p. 10 (13) 

Div. 7-220.1 1-M17 

136. A Further Analysis of the Precision of Range Corrector 

Settings for Small Monocular Range Finders (Progress 
Report 10), OEMsr-570, Jan. 26, 1943, p. 4 (7). 

Div. 7-210.14-M2 

137. Item Analysis No. V on the NRC Neurotic Inventory 

Tests Taken by the Submarine School Population (Prog- 
ress Report 11), OEMsr-570, Jan. 27, 1943, p. 3 (13). 

Div. 7-220.11-M18 

A Method for Approximating the Point of Zero Error in 
Ranging on Artificial Stereoscopic Targets Within a 
Reticle Field (Progress Report 12), OEMsr-1059, Jan. 29, 

1943, p. 6 (16). Div. 7-210.34-Ml 

A7i Analysis of the Precisio7i and Subject Variation of 
Ra7ige Corrector Setthtgs for S7nall Mo7iocular Range 
Finders (Progress Report 13), OEMsr-570, Feb. 15, 1943, 
p. 11 (7) (16). Div.7-210.14-M3 

Results for Selective Experi7nents at the U.S. Sub7na7i7ie 
Base, Nexc London, February 1943 (Progress Report 14), 
OEMsr-570, Feb. 27, 1943, p. 11 (13). Div. 7-220.1 1-M19 

Selective Data and Tank Perfor777ance at the U.S. Sub- 
7narine Base, Neic Lo7ido7i, Co7i7iecticut (Progress Report 
15), OEMsr-570, Mar. 8, 1943. p. 9 (13) Div. 7-220.1 1-M20 

The Effect of Illu7ninatio7i o7i the Precisio7i of Range 
Corrector Settings for S7nall Monocular Range Finde7s 
(Progress Report 16), OEMsr-570, Apr. 9, 1943. p. 2 
(7) (16). Div. 7-210.14-M4 

The Precisio7i of Mo7iocular Ra77ge Corrector Settwgs 
ivith a Patter77 of Vertical Lines, Studied as a Functio7i 
of the Separatio7i Between Lines, May 28, 1943, p. 2 
(7) (16). (Included in Ref. 31.) 

A Sta77dardizatio7i and Validatio77 of the Perso7ial hi- 
ventory: Psychiatric Criterion, OSRD 1606, June 24, 1943, 
p. 21 ' (13). APP-321-M1 

Distributions of Measures of Interpupillary Distance, 
OSRD 1341, Mar. 24, 1943, p. 9 (13). APP-121-M1 

Manual for the Adjust7nent and Operation of the Projec- 
tion Eiko7707neter, OSRD 1340, Mar. 29, 1943, p. 8 (13). 

APP-122-M1 


138. 

139. 

140. 

141. 

142. 

143. 

144. 

145. 

146. 


131. 

132. 




BIBLIOGRAPHY 


183 


147. An Instrument for the Measurement of Interpupillary 

Distance, OSRD 1372, Apr. 6, 1943, p. 7 (13). 

APP-653-M1 

148. Preliminary Suggestions for a Stereoscopic Spotting 
Trainer, OSRD 1374, Apr. 3, 1943, p. 9 (14). 

APP-654-M1 

149. The Use of a Template in Making the Interocular Set- 

ting on the Height Finder, OSRD 1373, Apr. 7, 1943, 
p. 5 (4). APP-652-M1 

150. Informal Report of Studies Conducted at Camp Davis, 

N. C., under the Auspices of the National Research 
Council, May 23, 1943, p. 6 (4) (13). 

151. The Adjustment of the M2 Trainer for Standard Testing, 

OSRD 1638, June 17, 1943, p. 3 (13). APP-652-M3 

152. The Design and Construction of a Height Finder Swi- 

shade, June 15, 1943, p. 7 (5). Div.7-210.2-M2 

153. The Relationship Beticeen Test Scores Obtained on the 

Single and Multiple Projection Eikonometers. OSRD 1789, 
Aug. 5, 1943, p. 4 (13). APP-122-M2 

154. A Model of the Optics of the Stereoscopic Height Finder, 

OSRD 1396, Apr. 26, 1943. p. 4 (14). APP-652-M2 

155. A New Motor Attachment for the M2 or the Mark H 

Stereoscopic Trainer, OSRD 1392, Apr. 26, 1943, p. 5 
(13). APP-654-M2 

156. A Report of Research on Selection Tests at the U.S. 
Submarine Base, Neiu London, Connecticut, June 28, 

1943, p. 65 (13). 

157. The Relation of Selection Test Scores to Tank Escape 
Performance: Submarine School, OSRD 3262, Jan. 31, 

1944, p. 7 (13). APP-413-M2 

158. A Stereomicrometer Device for Precision Measurements 

of Stereoscopic and J'ernier Acuities (Report 1), Lorrin 
A. Riggs and Roberta M. Daley, OEMsr-1059, Aug. 30, 
1943, p. 9 (7) (16). ' Div. 7-220.12-M20 

159. A Comparison of Various Visual Patterns for Use in Con- 

nection with the Internal Adjuster System of a Range 
Finder (Report 2). Lorrin A. Riggs and Roberta M. Daley, 
OEMsr-1059, Sept. 10, 1943, p. 19 (7) (16). 

Div. 7-210.1 12-M2 

160. A Preliminary Experiment on the Comparative Precisions 

of Settings Made ivith Four Stereoscopic Reticle Patterns 
(Report 3), Lorrin A. Riggs and Roberta M. Daley, 
OEMsr-1059, Nov. 10, 1943, p. 7 (16). (Included in 

Ref. 44). Div. 7-210.33-M3 

161. The Reliability of Reticle Inspection (Report 4), Roberta 

M. Daley and Clarence H. Graham, OEMsr-1059, Dec. 30, 
1943, p. 6 (16). (Included in Ref. 40.) Div. 7-210.32-Ml 

162. Preliminary Experiment on the Effects of Extraneous 

Stimuli (Imperfections) hi the Reticle Field Upon Pre- 
cision and Consistency of Stereoscofiic Performance (Re- 
port 5), R. L. Solomon and Clarence H. Graham, OEMsr- 
1059. Jan. 10, 1944, p. 8 (16). (Included in Ref. 40.) 

Div. 7-210.32-M2 

163. The Effect of Position of Fore and Aft Reticle Marks on 

Precision of Stereoscopic Settings. Preliminary Results 
(Report 6), Roberta M. Daley, Clarence H. Graham and 
Lorrin A. Riggs, OEMsr-1059, Feb. 8, 1944, p. 6 (16). 

(Included in Ref. 44.) Div. 7-210.33-M4 

164. Precision of Stereoscopic Settings as Influenced by Dis- 

tance of Target from a Fiducial Line (Report 7), Clarence 
H. Graham, Roberta M. Daley and R. L. Solomon, 
OEMsr-1059, Feb. 9, 1944, p. 5 (10). (Included in 

Ref. 44.) Div.7-210.33-M5 


165. Precisions Obtained xvith Fore and Aft Fiducial Marks 

at Different Apparent Depths but Constant Separation 
in the Stereoscopic Plane (Report 8), Roberta M. Daley, 
Lorrin A. Riggs and Clarence H. Graham, OEMsr-1059, 
Feb. 15, 1944, p. 2 (16). (Included in Ref. 44.) 

Div 7-210.34-M2 

166. Monocular vs Binocular Internal Adjuster Settings, Lor- 
rin A. Riggs, Feb. 16, 194i p. 1 (7). Div. 7-210.1 12-M4 

167. Item Analysis and Evaluation of the Scoring Stencil of 

the Personal Inventory, OSRD 3315, Feb. 14, 1944, p. 13 
(13). ' ' APP-412-M2 

168. An Investigation of Reticle Factors which Might Possibly 

Influence Precision of Stereoscopic Setting (Report 9), 
Roberta M. Daley, Clarence H. Graham and Lorrin A. 
Riggs, OEMsr-1059, Mar. 28, 1944, p. 9 (16). (Included 

in Ref. 44.) Div. 7-210.33-M6 

169. The Personal Inventory, Short Form (Format C): Deriva- 

tion and Preliminary Psychiatric Validation, OSRD 3390, 
Mar. 15, 1944, p. 11 (13). APP-321-M3 

170. A Mirror Stereoscopic Device for Simulating a Moving 
Target tVithin the Field of a Stereoscopic Height Finder 
(Report 10), C. G. Mueller and Lorrin A. Riggs, OEMsr- 
1059, May 4, 1944, p. 7 (16). (Included in Ref. 44.) 

Div.7-210.23-Ml 

171. Precisions and Constant Errors of Stereoscopic Settings 
as Influenced by Differences in Reticle Pattern, Reticle 

I Patterns Based on Service Reticles (Report 11), Lorrin A. 

I Riggs, C. G. Mueller and Roberta M. Daley, OEMsr- 

I 1059, May 9, 1944, p. 8 (16). (Included in Ref. 44.) 

Div.7-210.31-M3 

172. Methodological Considerations Having to do With the 

Type of Course Used in Experiments on Reticle Design 
and the Reliability of Experimenters' Determinations 
(Progress Report 12), Marion S. Borod, Roberta M. Daley, 
and others, OEMsr-1059, May 19, 1944, p. 11 (16). (In- 
cluded in Ref. 44.) Div. 7-210.31-M4 

173. The Influence of Tivo Reticle Patterns, the Navy Open 

Diamoxid and a Three-dot Pattern, on Precision of Ster- 
eoscopic Settings (Report 13), Lorrin A. Riggs and 
Roberta M. Daley, OEMsr-1059, May 26, 1944, p. 3 (16). 

(Included in Ref. 44.) Div. 7-210.31-M5 

174. Precisions and Constant Errors of Stereoscopic Settings 
as Influenced by Extraneous Stimuli in the Reticle Field: 
Experiments Involving Stereoscopic Movement and 
Tracking Errors (Report 14), Clarence H. Graham, 
R. L. Solomon, and C. G. Mueller, OEMsr-1059, June 3, 
1944, p. 6 (16). (Included in Ref. 40.) Div. 7-210.33-M7 

175. The Ability of Stereoscopic Observers to Signal Loss of 

Contact with the Reticle. Aided Ranging 07i a Diving 
Target (Report 15) , Clarence H. Graham, Lorrin A. 
Riggs, and others, OEMsr-1059, July 28, 1944, p. 13 (16). 

Div. 7-210.34-M3 

176. Results Obtained with Various Patterns of Opaque 

Reticle (Report 16), C. G. Mueller, R. L. Solomon, and 
Marion S. Borod, OEMsr-1059, Aug. 17, 1944, p. 17 (16). 
(Included in Ref. 44.) Div. 7-210. 31-M6 

177. Stereoscopic Performance on Different Reticles in the 

Absence of Fine Elevation Adjustments (Progress Report 
17), R. L. Solomon, Clarence H. Graham and Roberta 
M. Daley, OEMsr-1059, Sept. 11, 1944, p. 6 (16). (In- 
cluded in Ref. 44.) Div. 7-210.34-M4 

178. Note on False Fusiofi in Various Reticle Patterfis (Prog- 

ress Report 18), Roberta M. Daley and Clarence H. 
Graham, OEMsr-1059, Sept. 22, 1944, p. 10 (16) . (In- 
cluded in Ref. 44.) Div. 7-210.31-M7 


184 


BIBLIOGRAPHY 


179. Slereoscopic Perfonnaiice for Different Reticles as In- 

fluenced by Height of Adjustment Errors in Components 
of the Visual Fields (Progress Report 21),Lorrin A. Riggs, 
G. G. Mueller, and others, OEMsr-1059, Oct. 21, 1944, 
p. 15 (12) (16). (Included in Ref. 44.) Div. 7-210.34-M5 

180. Measures of Interpupillary Distance (Progress Report 

20), Lorrin A. Riggs, Roberta M. Daley, and F. A. Mote, 
OEMsr-1059, Oct. 16, 1944, p. 8 (13). (Included in 

Ref. 44.) Div. 7-230.2-Ml 

181. Stereoscopic Performance When a Stationary Target is 

Partially Obscured by Reticle Lines of Various Thick- 
nesses (Progress Report 22) , C. G. Mueller, Roberta M. 
Daley, and F. A. Mote, OEMsr-1059, Nov. 10, 1944, p. 5 
(16). (Included in Ref. 44.) Div. 7-210. 34-M6 

182. The Influence of Height of Adjustment Errors on the 

Precision and Accuracy of Ranging on Four Reticle Pat- 
terns (Progress Report 23), Clarence H. Graham, Lorrin 
A. Riggs, and others, OEMsr-1059, Nov. 16, 1944, p. 8 
(16). (Included in Ref. 44.) Div. 7-210.31-M8 

183. A7i Investigation of Possible Effects Attributable to 

Torsion in Stereoscopic Performance (Progress Report 
24), Clarence H. Graham, Lorrin A. Riggs, and others, 
OEMsr-1059, Dec. 5, 1944, p. 7 (16). (Included in 

Ref. 44.) Div. 7-230.2-M2 

184. Further Experiments on Height of Image Error Effect 

of Force and Aft Marks and a Horizontal Line. Pre- 
cisio7is and Accuracies of Height of Image Adjustment 
(Progress Report 25), Clarence H. Graham, Lorrin A. 
Riggs, and others, OEMsr-1059, Jan. 19, 1945, p. 9. 
(Included in Ref. 44.) Div. 7-210.34-M7 

185. Memo: Suggestions for Reticle Design and Related Mat- 
ters. Feb. 2, 1945, p. 3. (Included in Ref. 44.) 

186. Report in Summary of Research on the Design of Opaque 
Reticles, Feb. 13, 1945, p. 27. (Included in Ref. 44.) 

CALIFORNIA INSTITUTE OF TECHNOLOGY 

187. Range Finder Optical Systeins, J. A. Anderson, I. S. 
Bowen, and others, NDCrc-123.7, 1942, p. 33 (2). 

Div. 7-210.13-Ml 

188. Range Finder Optical Systems (Supplement) J. A. An- 

derson, I. S. Bowen, and others, NDCrc-123.7, 1942, p. 6 
(2). Div. 7-210.13-M2 

DARTMOUTH COLLEGE 

189. Progress Report No. I, General Problem, May 16, 1942, 

p. 19 (9). 

190. Progress Report No. 2, Aug. 1, 1942, p. 14 (9). 

191. Progress Report No. 3, Sept. 17, 1942, p. 8 (9). 

192. Data Supplement to Third Progress Report, Oct. 16, 

1942, p. 22 (9). 

193. The Effect of Fatigue on liinocular Spatial Localization 

(Final Report), Robert J. Beitel, Jr., S. Howard Bartley, 
and others, OEMsr-473, Feb. 27, 1943, p. 74 (9). 

Div. 7-220.16-M8 

194. The Effect of Fatigue on Binocular Spatial Localization 

(Progress Reports 1 - 4), Robert J. Beitel, Jr., Adelbert 
Ames, and Hermann M. Burian, OEMsr-473, May 16, 
1942 to Feb. 27, 1943, p. 10 (9). Div. 7-220.16-Ml 

EASl MAN KODAK COMPANY 

195. Acuity Tester. Description of a Multi-purpose Height 

Finder Training and Testing Instrumeyit, Jan. 29, 1942, 
p. 13 (14). (Included in Ref. 17.) 


196. Full-Field Coincidence Range Finder of 15-inch Base 

Provided icith Continuously Adjustable Range Compen- 
sation, May 21, 1942, p. 4 (15). (Included in Ref. 6.) 

Div. 7-210.17-M2 

197. Auto-Collimating Systems for Range Finders, Stephen 

M. MacNeille and F. M. Bishop, Problems DD-2492R 
and DD-2492S, OEMsr-56, June 9, 1942, p. 14 (17). 

Div.7-210.18-Ml 

198. Specifications for Proposed M-I Range Finder Trainer 

for U.S. Army, July 7, 1942, p. 5 (17). 

Div.7-220.213-M2 

199. Tests of 15-in. Range Finder, D. F. Lyman and F. M. 

Bishop, OEMsr-56, Problem DD-2492AA, Dec. 4, 1942, 
p. 26 (15). Div.7-210.17-M4 

200. A Proposed Optical Desigfi for Super Range Finder, 

Joseph Mihalyi, Contract Symbol 2489, Problem DD- 
1621, Apr. 15, 1943, p. 4 (17). Div. 7-210.1-Ml 

201. Range Finder Auto-Collimation an d Appl ications, Stephen 
M. MacNeille and F. M. Bishop, OEMsr-56, Problem No. 
DD-2492S, Apr. 24, 1943, p. 6 (17). Div. 7-210.18-M2 

202. Effect of Temperature Gradient on Mirror Flatness, 

Sept. 11, 1943, p. 6 (5). Div. 7-210.23-M2 

203. One Source of Error in the M-1 Height Finder, William 

A. Arnold, Stephen M. MacNeille, and F. M. Bishop, 
OFMsr-56, Problem DD-2492V, Oct. 7, 1942, p. 10 (5). 

Div.7-210.21-M2 

204. Summary of Reports on an Ortho-Pseudo Stereoscopic 
and Coincidence Height Fmder (10 Sections), p. 43 
(3) (17). (Included in Ref. 21.) 

205. Effect of Temperature Gradient on Deviatiofi of Range 

Finder Penta-Reflectors, Jan. 28, 1944, p. 15 (5). 

(Included in Ref. 41.) 

206. Operating Instructions and Description of the Eastman 

Fxperitnental Model 1 -Meter Infantry Range Finders 
T-25 and T-26, Joseph Mihalyi and F. M. Bishop, 
OEMsr-56, Feb. 1944, p. 12 (15). Div. 7-210.14-M5 

207. Distortions of Thin Slabs Due to Temperature Gradients 

(Report 55), ’W^illiam A. Arnold, OEMsr-952, May 1, 1944, 
p. 10 (5). (Included in Ref. 41.) Div. 7-210.12-Ml 

208. The 15-inch Full Field Superitnposed Image Type Close- 

Distance Range Finder, OEMsr-56, Apr. 7, 1944, p. 10 
(15). Div.7-210.17-M5 

209. Elimination of Thermal Effects in Pentareflectors (Re- 

port 56), William A. Arnold and Wayne G. Norton, 
OEMsr-952, July 12, 1944, p. 23 (5). (Included in 
Ref. 41.) Div. 7-210.21 -M5 

210. A Study of Range Errors in M-1 Height Finders Produced 

by the Pentareflectors and the Optical Bar Moutit, Martin 
S. Maier and William A. Arnold, OEMsr-952; Problem 
DD-2492NN, Aug. 18, 1944, p. 22 (5). (Included in 
Ref. 41.) Div. 7-21 0.2 1-M5 

211. Aji Ortho-Pseudo Stereoscopic 1 -Meter Base Range 

Finder, Joseph Mihalyi and F. M. Bishop, OEMsr-56; 
Problem DD-2492LL, Nov. 10, 1944, p. 4 (15). 

Div. 7-210.16-M2 

212. The 15\/2-Foot Superimposed Range Finder, Joseph 
Mihalyi and F. M. Bishop, OEMsr-56; Problem DD- 
2492HH, Nov. 10, 1944, p. 13 (17). Div. 7-210.19-M2 

213. Ortho-Pseudo Super Range Fmder, Joseph Mihalyi, Otto 

Wit tel, and Martin S. Maier, OEMsr-952; Problem DD- 
1621, Nov. 22, 1944, p. 24. Div. 7-210.16-M3 


RESTRICTED 


BIBLIOGRAPHY 


185 


214. A Proposed Polarizing Beam-Splitter, George J. Koch, 
OEMsr-952; Problem DD-1621, Nov. 1, 1944, p. 19. 

Div. 7-210.19-Ml 

FOXBORO COMPANY 

215. A Study of Factors Determining Accuracy of Tracking 

by Means of Handwheel Control (Report 1), OEMsr-453, 
Sept. 30, 1942, p. 56 (9). (Included in Ref. 33.) 

Div. 7-220.311 -Ml 

216. A Supplemental Study of Factors Determming Accuracy 

of Tracking by Means of Handwheel Control (Report 
2), OEMsr-453, Dec. 7, 1942, p. 38 (9). (Included in 
Ref. 34.) Div. 7-220.311 -M2 

217. Handwheel Speed and Accuracy of Tracking, (Report 3), 

OEMsr-453, May 12, 1943, p. 10 (9). (Included in 

Ref. 35.) Div. 7-220.3 11 -M3 

218. A Comparison of Three Stadia Ranging Reticles, Nov. 27, 

1943, p. 26 (16). 

219. Effects of Target Speeds and Rates of Turning on Ac- 

curacy of Direct Handivheel Tracking (Memorandum 13), 
OEMsr-453, July 6, 1944, p. 11 (10). Div. 7-220.31 1-M5 

220. Improvement in Direct, Aided and Velocity Tracking 

Through Magnification of Data Preseyitation (Memoran- 
dum 14), OEMsr-453, Aiig. 24, 1944, p. 14 (10). 

Div.7-220.312-Ml 

221. Simultaneous Hand and Foot Operation of Tracking and 

Ranging Controls (Memorandum 12), OEMsr-453, June 
27, 1944, p. 18 (15) (16). Div. 7-220.33-Ml 

222. Progress Report on Comparative Slewing Times with 

Optical Ring, Telescopic and Ring-Post Sights, Dec. 30, 
1943, p. 9 (10). (Included in Ref. 43.) 

223. Progress Report on Comparative Tracking Errors with 

Illuminated, Ring-Post, Telescopic and Optical Ring 
Sights, Jan. 20, 1944, p. 4 (10). (Included in Ref. 43.) 

224. Progress Report on Sleiving Times with Illuminated 

Ring, Ring-Post, Telescopic and Optical Ring Sights, 
Feb. 3, 1944, p. 5 (10). (Included in Ref. 43.) 

225. Progress Report on Comparative Tracking Errors with 
Illuminated, Ring-Post, Telescopic and Optical Ring 

^ Sights, Feb. 23, 1944, p. 5 (10). (Included in Ref. 43 J 

226. Simultaneous Tracking and Ranging with Hands and 
Feet Versus All Hand Control (Memorandum 15), 
OEMsr-453, Sept. 15, 1944, p. 12 (15). Div. 7-220.33-M2 

227. Single Versus Double Pedal Ranging ]Vhile Handle-Bar 

Tracking (Memorandum 16), OEMsr-453, Oct. 20, 1944, 
p. 14 (15). Div. 7-220.33-M3 

FR ANKFORD ARSENAL, FIRE CONTROL 
DESIGN DIVISION 

228. German 4-Meter Range Finder Model Em. 4 on R 40, 

Oct. 20, 1943, p. 15 (3) (16) (17). 

229. Initial Estimates of the Effect of Range Finder Accuracy 

on Battery Performance (Princeton Branch), Feb. 25, 
1943, p. 25 (1). 

230. Design of a H eight fmder Interpupillometer (Princeton 
Branch), Mar. 3, 1943, p. 6 (4). 

231. Basic Physical & Physiological Recfuirements for Satis- 
factory Range Finder Performance, Mar. 8, 1943, p. 20 
(17). 

232. Theory 6- Design of the Monofocle (Princeton Branch), 

Mar. 11, 1943, p. 7 (5). 


233. Basic Experiments of Importance in Range Finder Re- 
design (Princeton Branch), Mar. 31, 1943, p. 22 (17). 

234. The Computations of Branch Memorandum No. 2 
(Princeton Branch), Mar. 20, 1943, p. 33. 

235. Internal Stops for Height Finders (Princeton Branch), 
May 4, 1943, p. 4 (4). 

236. Oil Contamination from Ordinary Helium (Princeton 
Branch), May 22, 1943, p. 3 (5). 

237. Modification in a Special MI Height Finder (Princeton 

Branch), May 26, 1943, p. 8 (4). 

238. Performance of Student Observers (Princeton Branch). 
June 1, 1943, p. 13 (14). 

239. Comparison of Several Optical Designs for Reticle Type, 
Stereoscopic Range Finders (Princeton Branch), p. 25 
(3). 

240. Desirable Characteristics of Reticle Type, Stereoscopic 

Range Finders and their Relation to Several Proposed 
Optical Designs (Princeton Branch), p. 19 (3). 

241. Comparison of Range Finder Fields (Princeton Branch), 

July 8, 1943, p. 9 (2) (15). 

242. Comparison of Invert-Foreground and Invert-Sky Range 
Finder Fields for Ranging on Ground Targets (Princeton 
Branch), July 8, 1943, p. 8 (2) (15). 

243. IVedge-Check and Leveling Problems in the MI Height 

Finder (Princeton Branch), p. 35 (8). 

244. Analysis of Range Estimation Data (Princeton Branch), 

Sept. 8, 1943, p." 17 (15). 

245. Comparison of Trainers. Mark 4 and M6 (Princeton 
Branch), Jan. 12, 1944, p. 7 (14). 

246. Fire Control-Design Data for A. P. C. Projectiles (Prince- 
ton Branch), Oct. 16, 1943, p. 34 (15). 

247. Proposed Program of Study and Design of a Unified Field 

Modification of Height Finder MI (Princeton Branch), 
Feb. 17, 1944, p. 154^ (4) (17). 

248. The F Range Finder (Princeton Branch), Feb. 24, 1944, 
p.21 (17).^ 

249. Sensing Studies (Princeton Branch), May 12, 1944, p. 22 
(15). ^ 

250. A Modified Fire Control System for Gun Motor Carriage 

T-70 (Princeton Branch), May 10, 1944, p. 27 (15). 

251. Cold Weather Tests of Height Finder Performance 

(Princeton Branch), June 14, 1944, p. 22 (5). 

252. The F Range Finder for Tank Use (Princeton Branch), 
June 26, 1944, p. 10 (17). 

253. Integrated Fire Control for Armored Vehicles (Princeton 
Branch), June 27, 1944, p. 6 (15). 

254. Summary of Work Done on Development of Fire Control 
for Armored Vehicles (Princeton Branch), June 27, 1944, 
p.5 (15). 

255. Visual Range Estimation. Direct and Differential (Prince- 
ton Branch), June 19, 1944, p. 16 (15). 

256. Visual Lead Estimation (Princeton Branch), June 19, 
1944, p. 8. 

257. Level Collimator (Princeton Branch), June 1944, p. 15 

( 8 ). 

258. Final Report on the M1E9 Height Finder Princeton 

Branch), June 1944, p. 31 (17). 

259. Tests of Rangefinders T 23 and T 26 (Princeton Branch), 
June 13, 1944, p. 7 (15). 


RESTRICTED 


186 


BIBLIOGRAPHY 


260. FiiU-Lme Reticles in Stereoscopic Trainer M6 (Princeton 
Branch) 1944, p. 37 (16). 

261. Tests xvith Trainers Mark 4 and M6 (Princeton Branch), 
1944, p. 30 (2) (16). 

262. The Modified Stereo Trainer M6—An Experimental In- 

strument Providing Stereoscopic, Binocular and Monocu- 
lar Coincidence Rangwg on Real and Simulated Ground 
Targets (Princeton Branch), 1944, p. 27 (2). 

263. The Measurement of Focus Difference in the Ml Height 

Finder (Princeton Branch), 1944, p. 36 (5). 

HARVARD FATIGUE LABORATORY 

264. Visual Problems in Fire Control Progress Report of 

Project 10. Ross A. McFarland, Alfred H. Holway, and 
others. Feb. 9, 1942 (10). Div.7-220.1-M3 

265. Report, Fel)ruary 1942 (4). 

266. Posture and Stereo Acuity, Ross A. McFarland, Alfred H. 
Holway, and others, Apr. 24, 1942, p. 37 (9). 

Div. 7-220. 12-M4 

267. The Precision Consistency and Accuracy of Visual Range 

Observations, Ross A. McFarland, Alfred H. Holway, and 
others. May 14, 1942, p. 71 (2), (6). Div. 7-220.12-M6 

268. Altered Posture and Stereo Acuity, June 20, 1942, p. 17 

(9). ' Div. 7-220.12-M8 

269. The Limits of Binocular Fusion. Target Size, OEMsr-555, 

June 20, 1942, p. 5 (9). Div. 7-230.21-M3 

270. The Limits of Binocular Fusion. Binocular Vergence, 
OEMsr-555, June 20, 1942, p. 6 (9). Div. 7-230.21-M2 

271. Apparent Distance, Binocular Vergence and Target Size, 
OEMsr-555, June 20, 1942, p. 4 (9). Div. 7-230.21-Ml 

272. Apparent Size and Binocular Vergence. OEMsr-555, June 

20, 1942, p. 9 (9). Div. 7-230.21 -M4 

273. Loss of Sleep, Benzedrine and Stereo Acuity. June 20, 

1942, p. 6 (9). Div. 7-220.12-M12 

274. The Effects of Metrazol on Stereo and Vernier Acuity, 

June 20, 1942, p. 7 (9). Div. 7-220.12-Mll 

275. The Effect of Exercise on Stereo and Vernier Acuity, 

June 20, 1942, p. 7 (9). Div. 7-220.1 2-M 10 

276. Startle, Pupil Size, Stereo and Vernier Acuity, June 20, 

1942, p. 6 (9) Div.7-220.12-M9 

277. The Effects of Variations in Blood Sugar on Stereo and 
Vernier Acuity, June 20, 1942, p. 8 (9). 

Div.7-220.12-M13 

278. Low Oxygen, Low Illumination, Stereo and Vernier 

Acuity, June 20, 1942, p. 8 (9). Div. 7-220.12-M14 

279. The Effects of Hyperventilation on Stereo and Vernier 

Acuity, June 20, 1942, p. 3 (9). Div. 7-220.12-M15 

280. Psychophysiological and Psychophysical Studies of Stereo 
and Vernier Acuity, Ross A. McFarland, Alfred H. Hol- 
way, and others, June 20, 1942, p. 6 (2). 

Div.7-220.12-M16 

281. An Apparatus for Measuring Stereo and Vernier Acuity, 

June 20, 1942, p. 3 (14). ' Div. 7-220.12-M7 

282. Perspectwe Parallax Lines of Sight and Stereo Range 
Observations, .Apr. 16, 1943, p. 116 (4) (13). 

283. Stereoscopic Acuity and Telescopic Vision, (to appear 

ca. June 1945), p. ca. 250 (2) (6). Div. 7-220.12-M21 


HARVARD PSYCHO-EDUCATIONAL CLINIC 

284. Tests of Stereoscopic Vision for the Selection of Range 

Finder Operators, ^Valter F. Dearborn, Philip W. John- 
ston, and others. May 1, 1942, p. 31 (13). 

Div. 7-220.14-M2 

285. The Dearborn- Johnston Test for Depth Perception, p. 

19 (13). Div.7-220.13-M7 

286. Tests of Stereoscopic Vision for the Selection of Range 

Finder Operators. Experiment with 6Sth Coast Artillery 
Group, June 26, 1942, p. 10 (13). Div. 7-220.14-M7 

287. Some Correlations, June 26, 1942, p. 5 (13). 

288. Tests of Stereoscopic Vision for the Selection of Range 

Finder Operators. Report on Work at Camp Davis, June 
26, 1942, p. 3 (13). Div. 7-220.14-M6 

289. Tests of Stereoscopic Vision for the Selection of Range 

Finder Operators. Recommendations as to Use of 
Hoxvard-Dolman Apparatus, June 26, 1942, p. 2 (13). 

Div. 7-220.14-M8 

290. Validity of Vectographic Pursuit Apparatus, June 26, 
1942, p. 6 (13). 

291. Vectograph Pursuit Apparatus and Heterophorias, June 
26, 1942, p. 1 (13). 

292. Tests of Stereoscopic Vision for the Selection of Range 

Fmder Operators. The Wulfeck Test, June 26, 1942, p. 3 
(13). Div. 7-220.14-M5 

293. The Relation of M-H and Dearborn- Johnston Test Scores 

to Graduation Standing at the Rangefield School, Navy 
Yard, Washingtoxx, D. C., Jan. Class. 1943, p. 25 (13). 

Div. 7-220.15-M10 

294. The Dearborn-Johxxston Test of Stereoscopic Vision, p. 

17 (13). Div.7-220.13-M8 

295. Reliability of the Dearborn Ixistruxnents for Testing 

Stereoscopic Vision, OEMsr-555, Mar. 24, 1943, p. 5 (13). 

Div. 7-220.13-Ml 

296. Practice Effect of the Dearborn Instriunents for Testing 

Stereoscopic Vision, OEMsr-555, Mar. 25, 1943, p. 5 (13). 

Div. 7-220.13-M2 

297. The Relation of Dearborn Stereoscopic Test Scores to 

Validatioxi Scores of Class V at Camp Davis, OEMsr-555, 
p. 36 (13). Div.7-220.13-M3 

298. The Relation of Dearborn Stereoscopic Test Scores to 

Validation Scores of Class VI at Camp Davis, OEMsr-555, 
Mar. 31, 1943, p. 22 (13). Div. 7-220.13-M4 

299. The Administration and Standardization of the Dearborn 

Vectographic Pursuit Test, OEMsr-555, Apr. 8, 1943, p. 
13 (13). Div.7-220.13-M5 

300. Miscellaneous Notes on the Dearborn Tests of Stereo- 
scopic Vision, OEMsr-555, Apr. 12, 1943, p. 12 (13). 

Div. 7-220.13-M6 

HMS EXCELLENT 

301. Selection of Stereoscopic Rangetakers, Mar. 15, 1944, p. 
76 (13). 

302. The Effect of Lack of Parallelism of the Emergent Rays 
on the Vertical Meridian on Mean Coxisistency, Apr. 20. 
1944, p. 15 (8). 

303. Training of Stereoscopic Range Takers, Dec. 25, 1942, p. 
44 (14). 

304. The Effect of Height of Image Error on Mean Consist- 
ency in Stereoscopic Rangepxxders, Sept. 11, 1944, p. 25 
( 12 ). 


K^ESIRICTED 

— — — 


BIBLIOGRAPHY 


187 


305. A Study of the Available Methods of Removing Height 
of Image Error in Stereoscopic Rangefinders, Sept, 12, 
1944, p. 13 (12). 

HOWE LABORATORY OF OPHTHALMOLOGY 

30(). Experiments Determining the Effect of the Relative Posi- 
tion of Target and Reticle on Dial Readings in a Mark 
II Stereoscopic Training Instrument, Feb. 1942 (10). 

Div. 7-220.212-Ml 

307. Experiments Comparing Results Obtained with a Mark 
II Stereoscopic Training Instrument and those Obtained 
in Free Space, Feb. 9, 1942, p. 6 (10). Div. 7-220.212-M2 

308. Relative Effectiveness of Make-and-Break and Contmu- 
ous Tracking in Range at Carious Rates of Change of 
Angular Disparateness, June 22, 1942, p. 23 (12). 

Div. 7-220.34-Ml 

309. An Investigation of the Interval of Time Elapsing Be- 

tween the Making of Range and Signal that Range has 
been Made, June 22, 1942, p. 12 (9). Div. 7-220.12-M17 

310. An Investigation of the Amount of Practice Advisible in 

Ranging on a Simulated Diving Aeroplane Target, June 
22, 1942, p. 14 (14). Div. 7-220.2-Ml 

311. Retention of Stereoscopic Ability in the Absence of Prac- 
tice, June 23, 1942, p. 8 (9). Div. 7-220.17-Ml 

312. A Comparison of the Accuracy of Stereo Rarighig on Tar- 
gets of Different Types, June 23, 1942, p. 11 (12). 

Div. 7-220.2 13-Ml 

313. Experiments Attempting to Analyze Out Some of the 

Optical Factors Involved in Real Haze, Dec. 26, 1940, p. 
10 (11). Div.7-210.11-Ml 

314. A Comparison of the Howard-Dolman and Verhoeff Size 

Confusion Tests of Stereoscopic Ability, Elek Ludvigh, 
Nov. 27, 1941, p. 6 (13). Div. 7-220.12-M2 

IOWA STATE COLLEGE 

315. A Study of Antiaircraft Tracking (Final Report), John 

V. Atanasoff, Harold V. Gaskill, and others. OEMsr-165, 
Sept. 1942, p. 59 (10). Div. 7-220.34-M3 

316. A Study of Antiaircraft Tracking (Supplement to Final 
Report), John V. Atanasoff, Harold V. Gaskill, and others, 
OEMsr-165, Sept. 1942, p. 15 (10). Div. 7-220.34-M4 

317. A Study of Antiaircraft Tracking (Supplementary Report 
II), John V. Atanasoff, Harold V. Gaskill, and Sam Leg- 
void, OEMsr-165, Sept. 1942, p. 7 (9) (10). 

Div. 7-220.34-M5 

NAVAL INSPECTOR OF ORDNANCE 

318. Range Finders Mark 58 and Mark 63, Model 1 (formerly 

Mark 61 Model 1 ) Determination of the Effects of Helium 
Changes, report from Naval Inspector of Ordnance at 
Bausch & Lomb Optical Company to Chief of the Bu- 
reau of Ordnance, Jan. 11, 1943, p. 21 (4) (5). 

319. Recommendations for Selection and Training of Fire 
Controlmen Third Class (R), Lt. Comdr. A. L. Shepherd, 
Feb. 18, 1943, p. 46 (13) (14). 

NATIONAL BUREAU OF STANDARDS 

320. A Stereoscopic Telemeter, Sept. 22, 1941, p. 9 (5). 

321. The Performaiice of a Height Finder with Eccentric 

Apertures, Nov. 21, 1941, p. 4 (4). 

322. The Error Structures of a Range or Height Finder, Jan. 
9, 1945, p. 6 (8). 


323. A Qiiantitative Evaluation of a Psychological Error of the 
Stereoscopic Optical Telemeter, Jan. 11, 1945, p. 20 (8). 

OHIO STATE UNIVERSITY 

324. The Accuracy and Precision of Internal Adjuster Settings 

Using Single Bars ayid Parallel Bars as Targets, G. A. Fry, 
Charles S. Bridgman, and others, OEMsr-637, 1943, p. 
28 (7). Div. 7-210.1 12-M6 

325. Effects of the Intermittent Visibility of Low Contrast 

Targets on Stereo Range Measurements, G. A. Fry, 
Charles S. Bridgman, and V. J. Ellerlirock, OEMsr-637, 
June 23, 1943, p. 1.32 (11). Div. 7-210.1 1-M8 

326. The Relation of Chromatic Aberration and Dispersion 

of the Eye to the Blurredness of an Object Seen Through 
the M-1 Height Finder, G. A. Fry, Charles S. Bridgman, 
and others, OEMsr-637, July 1, 1943, p. 49 (11). 

Div. 7-230.1 -M2 

327. Relation of Blurredness to the Precision of Stereo Set- 

tings, G. A. Fry, Charles S. Bridgman, and others, OEMsr- 
637, July 1, 1943, p. 65 (11). Div. 7-210.1 1-M6 

328. Stereo Acuity in Relation to Accommodation and Verg- 
ence (Report 6), G. A. Fry, Charles S. Bridgman, and 
M. J. Allen, OEMsr-637, Aug. 18, 1943, p. 24 (11). 

Div.7-220.12-M19 

329. The Dependence of Cyclophoria on Eye and Head Posi- 
tion (Report 7), G. A. Fry, Charles S. Bridgman, and M. 
J. Allen, OEMsr-637, Aug. 25, 1943, p. 35 (9). 

Div. 7-220.19-M3 

330. Speed and Accuracy in Spotting with a Height Finder 
(Report 9), G. A. Fry, Charles S. Bridgman, and others, 
OEMsr-637, Sept. 15, 1943, p. 44 (12). Div. 7-220.19-M4 

331. Means for Measuring and Compensating Chromastereop- 
sis with Special Reference to the Use of the M-1 Height 
Finder, G. A. Fry, Charles S. Bridgman, and others, 
OEMsr-637, Sept. 20, 1943, p. 78 (11). Div. 7-230.1-M3 

332. Maintaining Contact on a Moving Target with a Stereo- 
scopic Range Finder, G. A. Fry, Charles S. Bridgman, and 
others, OEMsr-637, Nov. 15, 1943, p. 79 (12). 

Div. 7-210.15-M3 

333. A Combinatioti Tracking Telescope ayid Coyitrast Meter 

for a Stereoscopic Range Finder, G. Fry, Charles S. 
Bridgman, and others, OEMsr-637, Nov. 30, 1943, p. 30 
(11). Div.7-210.15-M4 

334. The Chroyyiatic Dispersion of the Huynayi Eye ayid its 

Possible Difluence oyi Stereoscopic Rayyge Finding, G. A. 
Fry, Charles S. Bridgman, and others, OEMsr-637, July 
28, 1942, p. 29 (11). Div. 7-230.1-Ml 

335. Effect of Atyyiospheric Scattermg Upon the Appearance 

of a Dark Object Agamst a Sky Background, G. A. Fry, 
and Charles S. Bridgman, OEMsr-637, July 28, 1942, p. 
23 (11). Div.7-210.111-Ml 

336. The lyiffueyyce of Atynospheric Scattering in Range Find- 

ing, G. A. Fry and Charles S. Bridgman, OEMsr-637, Aug. 
21, 1942, p. 46 (11). Div. 7-210.1 11-M2 

337. Effect of Reduced Coyitract Between the Target and its 

Backgrouyid m Stereo Range Finding, G. A. Fry, Charles 
S. Bridgman, and V. E. Ellerbrock, OEMsr-637, p. 66 
(11). Div.7-210.33-M8 

338. The Effect of Blurredness of Target on Stereo Judgmeyits, 

G. A. Fry, V. E. Ellerbrock, and Charles S. Bridgman, 
Nov. 27, 1941, p. 45 (9). Div. 7-220.12-Ml 


RESTRICTED 


188 


BIBLIOGRAPHY 


OPERATIONS RESEARCH GROUP [ORG] 

339. Comparison of Methods of Ranghig for Tank Gunnery 
on Stationary Targets at Long Range, Jan. 1943, p. 13 
(15). 

340. Estimation of Visual Range, and Corrections to Range 

Using Visual Observation and Sights with Magnification 
2x, 3x. 4x and 6x, Apr. 15, 1943, p. 2 (15). 

POLAROID COMPANY 

341. Polaroid Range Finding Sight Mark I and II, p. 6 (15). 

(Included in Ref. 6.) 

342. Polaroid Binocular Rangefinding Attachment Mk. I, p. 1 
(15). (Included in Ref. 6.) 

PRINCETON LABORATORY, FORT MONROE 

343. Preliminary Tests of Temperature Effects on the Stereo- 
scopic Height Finder, June 5, 1941, p. 21 (5). 

344. Comparison of 12 and 24 Power in Ranging on Fixed 

Targets, Aug. 18, 1941, p. 13 (6). 

345. The Use of Helium as a Charge for the M-1 Height 

Finder (Report 4), NDCrc-186, Aug. 18, 1941, p. 25 
(5). Div. 7-210.21-Ml 

346. Photogrammetry, Aug. 18, 1941, p. 14 (14). 

347. Effects of Thermal Instability on Height Finder Accuracy, 
Dec. 9, 1941, p. 34 (5). 

348. The Use of External Targets in Range Finder Adjust- 
ment and Training, Jan. 12, 1942, p. 5 (7). 

349. The Elimination of Need for Frequent Adjustment of 
Range Finders, July 13, 1942, p. 25 (7). 

350. Height Finder Performance at One-Inch Aperture, Mar. 
26, 1942, p. 6 (4). 

351. Mihalyi Comparative Test, July 13, 1942, p. 30 (3) 

(17). 

352. Method of Charging Height Finders MI and M2 ivith 
Helium, Mar. 26, 1942, p. 16 (5). 

353. Comparative Test of Coincidence and Stereoscopic 
Height Finders (Report 12), Merrill M. Flood, NDCrc- 
186, Revised Aug. 12, 1942, p. 18 (3). Div. 7-210.2-Ml 

354. Description of Instruments for Use in the Selection of 

Stereoscopic Range Finder Operators (Report 13), Mer- 
rill M. Flood and Thornton C. Fry, NDCrc-186, May 14, 
1942, p. 3 (13). Div. 7-220.14-M3 

355. Manual for Use in the Selection of Stereoscopic Range 

Finder Operators (Report 14), Merrill M. Flood, NDCrc- 
186, June 1, 1942, p. 34 (13). Div. 7-220.14-M5 

356. Supplement to: Manual for Use in the Selection of 

Stereoscopic Range Finder Operators (Report 14a), Mer- 
rill M. Flood, NDCrc-186, Dec. 15, 1942, p. 25 (13). 

Div. 7-220.14-M12 

357. Validatioii and Standardization of Tests Used in the 
Selection of Stereoscopic Rangefinder Operators, July 13, 
1942, p. 64 (13). 

358. Studies of Range Finder Errors Caused by Temperature 
Instability, July 13, 1942, p. 67 (5). 

359. The Effect of Reduced Power and Aperture on Height 
Finder Performance on Aerial Targets, July 13, 1942, p. 
22 (4) (6). 

360. Training Stereoscopic Height Finder Observers: The 

Relative Effectiveness of the Ml Height Finder, the 
M2 Trainer and the Eastman Trainer, July 30, 1942, 
p. 45 (14). 


361. Comparative Performance of Mickey and the Ml Stereo- 
scopic Height Finder in Ranging on Aerial Targets, Aug. 
12, 1942, p. 18 (3). 

362. A Comparative Test of Photogrammetric and Photothe- 
odolite Methods of Locating a Friendly Aerial Target, 
Aug. 12, 1942, p. 25 (14). 

363. Manual on the Determination of True Target Position 
by Phototheodolite, Aug. 20, 1942, p. 169 (14). 

364. Reduction of Perspective Error by the Operation of 

Height Finders at Reduced Aperture, Sept. 8, 1942, p. 
31 (4). 

365. An Elementary Discussion of Perspective Error in Range 
Finders, Dec. 4, 1942, p. 7 (4). 

3G6. Comparisons of Precision at 12 and 24 Power of Observa- 
tions Taken until the M-1 Height Finder for Fixed 
Ground Targets and Moving Aerial Targets, Merrill M. 
Flood, Report 23, RS 41, NDCrc-186, Oci 13, 1942, p. 16 
(6). Div.7-210.15-Ml 

367. Descriptive Summary of Errors Made ivith the Ml Height 

Finder During Field Tests at Fort Monroe, Report 24, 
RS 54, Oct. 28, 1942, p. 65 (14). Div. 7-210.11-M5 

368. Learning Rates and Training Program of Stereoscopic 
Height Finder Observers (Report 25), Merrill M. Flood, 
NDCrc-186, Dec. 15, 1942, p. 26 (14). Div. 7-220.14-M10 

369. The Retention of Helium by Stereoscopic Range Finders 
Under Service Conditions, Dec. 24, 1942, p. 37 (5). 

370. Stereoscopic Testing Center Progress Reports. 10 Reports 

from Fort Eustis; 1 Report from Camp Wallace, Septem- 
ber 1, 1942, Dec. 15, 1942, p. 321 (13). 

370a. True Target Position Location, p. 117 (14). 

371. Training Methods, p. 4 (14). 

372. Tracking Errors, p. 14 (10). 

373. Effect of Weather Factors on RCS Used, Apr. 1, 1941, p. 

6 ( 11 ). 

374. Study of Net Correction to RCS and Observers UOE 

from Fixed Target Data, Apr. 1, 1941, p. 4 (7). 

375. Study of Net Correction to RCS and Observers UOE 

from Moving Target Data, Apr. 1, 1941, p. 1 (7). 

376. Study of Means and Variations of RCS Readings from 

Experiments lA, February 12-13, 1941, Courses 1, 3, 5, 
7, 9, Apr. 1, 1941, p. 5 (7). 

377. Study of Means and Variations of RCS Readings from 
Experiments 3A and 3B, Apr. 1. 1941, p. 4 (7). 

378. Study of Variances of RCS Readings from Experiments 
5 A, 5B, 6 A, 6B, Apr. 1, 1941, p. 3 (7). 

379. Study of Means and Variances of RCS Readings from 
Exj)eriments 7 and 8, Apr. 1, 1941, p. 4 (7). 

380. Study of Means and Variances of RCS Readings from 
Experiment A, Apr. 1, 1941, p. 4 (7). 

381. Study of Means and Variances of RCS Readings from 

Experiments 11, 12, 13, and 14, Apr. 1, 1941, p. 7 (7). 

382. Study of Variation due to RCS Determinations, from 

Experiments 3-6 (February 18-21), May 6, 1941, p. 2 (7). 

383. Study of Variation of RCS due to Observer, Instrument 
and Method of Contacts. Experiments 18-22, May 6, 1941, 
p. 5 (7). 

384. Study of RCS and Net Corrections to RCS under Con- 
stant Temperature, May 6, 1941, p. 2 (7). 

385. Influence of Errors in RCS on R. F. Errors, May 6, 1941, 
p.6 (7). 



BIBLIOGRAPHY 


189 


386. Analysis of Variance of Net Correction to RCS Experi- 

77ients 3 and 4, May 6, 1941, p. 2 (7). 

387. The Effect of Internal Target Position on RCS, May 6, 
1941, p. 4 (7). 

388. Comparison of Binocular and Monocular RCS Settings 

(Report on Experiment K5), June 1, 1941, p. 3 (7). 

389. Accuracy of Single RCS Determinations, June 1, 1941, 
p.5 (7). 

390. Distribution of Errors RCS Readings, Apr. 1, 1941, p. 2 

( 7 ). 

391. Distribution of Errors. R. F. Readings on a Fixed Target, 
Apr. 1, 1941, p. 3 (14). 

392. Distribution of Errors. Distribution of R. F. Readings 
when Different Methods of Contact are Used from Data 
Taken on February 15, 1941. One course, 5 men, Apr. 1, 
1941, p. 3 (12) (14). 

393. Distributioji of Errors. Error Variances and Standard 
Errors from Fixed Target Data. Experiynents 23-33, May 

6, 1941, p. 2 (14). 

394. Distribution of Errors. Accuracy of R. F. Readings on 

Different Targets, June 1, 1941, p. 3 (14). 

395. Consistency of Meayi Readings. R. F. Readings on Fixed 
Targets from Data Taken on January 30, 1941, Apr. 1, 
1941, p. 4 (14). 

396. Consistency of Mean Readings. R. F. Readings on Fixed 

Target from Experiment lA, February 12-13, 1941. 

Courses 1, 3, 5, 7, 9, Apr. 1, 1941, p. 3 (14). 

397. Consistency of Mean Readings. R. F. Readings on Fixed 

Target from Experiment IB, February 12-13, 1941. 

Courses 2, 4, 6, S, 10, Apr. 1, 1941, p. 3 (14). 

398. Consistency of Mean Readings. R. F. Readings on Fixed 

Target from Experiment 2 A and 2B, February 17, 1941, 
Apr. 1, 1941, p. 4 (14). 

399. Consistency of Mean Readings. R. F. Readings on Fixed 
Target from Experiment 3A and 3B. February 18, 1941, 
Apr. 1, 1941, p. 4 (14). 

400. Consistency of Mean Readings. R. F. Readings on Fixed 
Target from Experiments 4 A and 4B, February 19, 1941, 
Apr. 1, 1941, p. 4 (14). 

401. Consistency of Mean Readings. R. F. Readings on Fixed 
Target from Experiments 5 A and 5B, February 20, 1941, 
Apr. 1, 1941, p. 4 (14). 

402. Consistency of Mean Readings. R. F. Readings on Fixed 
Target from Experiments 6A and 6B, February 21, 1941, 
Apr. 1, 1941, p. 4 (14). 

403. Consistency of Mean Readings. R. F. Readings on Fixed 

Target from Experiments 2-10, Apr. 1, 1941, p. 1 (14). 

404. Consistency of Mean Readings. R. F. Readings on Fixed 
Target from Experiments 5 and 6, February 20-21, 1941, 
May 6, 1941, p. 3 (14). 

405. Consistency of Mean Readings. R. F. Readings on Fixed 
Target from Experiments 7 and 8. February 24-25, 1941, 
May 6, 1941, p. 3 (14). 

406. Cotisistency of Mean Readings. R. F. Readings on Fixed 

Targets from Experiment 15 (Coast Artillery School 
Data), March 15, 1941, May 6, 1941, p. 3 (14). 

407. Study of Means and Dispersion of R. F. Readings from 
Data Taken on January 30, 1941. Tiuo Men, Two Instru- 
ments, Apr. 1, 1941, p. 2 (14). 

408. Study of Means and Dispersions of R. F. Readings from 
Experiment lA, February 12-13, 1941. Courses 1, 3, 5, 

7, 9, Apr. 1, 1941, p. 2 (14). 


409. Study of Means and Dispersions of R. F. Readings from 
Experiment IB, February 12-13, 1941. Courses 2, 4, 6, 
5, it?, Apr. 1, 1941, p. 3 (14). 

410. Study of Means and Dispersions of R. F. Readings from 

Experiments 2 A and 2B, Apr. 1, 1941, p. 5 (14). 

411. Study of Means and Dispersio^is of R. F. Readings from 

Experiments 3A and 3B, Apr. 1, 1941, p. 4 (14). 

412. Study of Means and Dispersions of R. F. Readings from 
Experiments 4A and 4B, Apr. 1, 1941, p. 4 (14). 

413. Study of Means and Dispersions of R. F. Readings from 

Experiments 5A and 5B, Apr. 1, 1941, p. 4 (14). 

414. Study of Means and Dispersions of R. F. Readings from 

Experiments 6A and 6B, Apr. 1, 1941, p. 3 (14). 

415. Study of Means and Dispersions of R. F. Readings from 

Experiments 7 A, 7B, 8A, and 8B, Apr. 1, 1941, p. 4 (14). 

416. Study of Means and Dispersions of R. F. Readings from 

Experiments 9 and 10, Apr. 1, 1941, p. 5 (14). 

417. Study of Means and Dispersions of R. F. Readings from 

Experiments 11, 12, 13, and 14, Apr. 1, 1941, p. 6 (14). 

418. Preliminary Study of Trends in H. F. Readings from 
Data Taken during August and September 1940, Apr. 1, 

1941, p. 10 (14). 

419. The Initial Observations on Aerial Courses, Sept. 14, 1942, 

p. 10 (14). 

420. Aerial Correlations on Test III Aerial Courses, Sept. 18, 

1942, p. 12 (14). 

421. Cojnparison of NDRC Aerial Courses with Similar 

C. A. B. Courses, Nov. 12, 1942, p. 12 (14). 

422. Effect of Method of Cojitact on R. F. Readings on Fixed 
Target from Data Taken on February 15, 1941. Clock 
Tower. One course, 5 teams, Apr. 1, 1941, p. 2 (12). 

423. Stereoscopic Height Finder Accuracy and Calibrations for 
Observer, Instrument, and Target. Analysis of Rangmg 
on fixed Targets, May 6, 1941, p. (14). 

424. Calibration of Man, Instrument and Target, from Data 

of Experiment 31, April 14, 1941, May 6, 1941, p. 4 (14). 

425. Data frotn Fixed Target Experiments 23-33, May 6, 1941, 
p. 9 (14). 

426. Effect on Curve B of Use of Approximate Formula for 

UOE, June 1, 1941, p. 5 (11). 

427. Changes in Curve B with time, June 1, 1941, p. 9 (11). 

428. Haze Effects and a Haze Meter, July 1, 1941, p. 2 (11). 

429. Diffuseness and Lack of Contrast in the Attnosphere, 
and their Possible Effects o7i Ra77ge Readings, Apr. 18, 
1942, p. 10 (11). 

430. Cha7iges in Internal Adjuster Readings at Range 00 luith 

Increasing Elevatio7i (Report on Experiment M2), June 
1, 1941, p. 5 (5). 

431. Variatio7i in Monocular Settings m Relation to RCS and 

Te7nperature (Report on Experiment Kl), June 1, 1941, 
p. 1 (5). 

432. Temperature Characteristics of Different I7istru77ie77ts. 

Report 071 Experiirient K6, June 1, 1941, p. 7 (5). 

433. Ther7nocouple Distallations in Ml Height Fmders, June 

1, 1941, p. 7 (5). 

434. Temperature and RCS from Fixed Target Experbnents, 
June 1, 1941, p. 7 (5). 

435. Changes in RCS with Chaiige m Angle of Elevatio7i, 

Co7npariso7i of Nitroge7i-filled a7id Helium-filled histru- 
7nent, June 1, 1941, p. 2 (5). 


■RESTRICTED 


190 


BIBLIOGRAPHY 


436. Thermocouple Measurements of Internal Temperatures, 

June 1, 1941, p. 6 (5). 

437. Filling Instruments Ml No. 158 a7id No. 68 with Helium, 

June 1, 1941, p. 1 (5). 

438. Thermocouple Measurements of Temperature Distribu- 

tio7i, I77stru77ie77t 158 (5). 

439. RCS Deter77ii7iations on Heliu77i-filled and Nitrogen-filled 

Instru77ie7its, July 1, 1941, p. 5 (5). 

440. Effects of Circulalmg Gas withm Height Fmder on hi- 
ter7ial Te77iperatures - Prelwihiary Report, July 1, 1941, 
p. 3 (5). 

441. Effects of Te7nperature Stratificatio7i on Height Finder 

Accuracy, Aug. 1, 1941, p. 4 (5). 

442. Teclmique for Testing a7id Flushing Heliu7n-charged 

Height Finders, Jan. 29, 1942, p. 13 (5). 

443. Experi77ie7its to Deter7nine Qua7ititative Relatio7is be- 
twee7i Thermal and Optical Co7iditio7is m an M2 Height- 
Finder, Apr. 28, 1942, p. 12 (5). 

444. Refractio7i m the Mam Path of a Modified M2 Height' 

Fmder and the Use of Ther7nocouples m Determinmg 
RCS, Apr. 18, 1942, p. 15 (5). 

445. The Emetics of the War7img Process in a Modified M2 

Height Fmder (Preliminary Report) , Apr. 18, 1942, 
p. 8 (5). 

446. Refraction in the hiternal- Adjuster Path of a Modified 

M2 Height Finder and the Use of Thermocouples to 
Deter7nine True Range, Apr. 18, 1942, p. 17 (5). 

447. Preparatio7is for Expe7iments to Deter77ii7ie the Effect 
of Solar Radiatio7i on the Optical Co7iditio7is m Height 
Fmders, July 25, 1942, p. 2 (5). 

448. Heliimi Reie7itivity of Raiige Finders and Height Find- 
ers in the Field, July 2, 1942, p. 12 (5). 

449. A Study of the Effect 07i Height Fmder Calibration of 
the Use of Heliu7n as a Chargmg Gas, Sept. 14, 1942, 
p.5 (5). 

450. The Oliver Helium Charging Method, Sept. 8, 1942. 
p.8 (5). 

451. Stratification Tests at East7na7i Kodak Pla7it, Oct. 24, 
1942, p.8 (5). 

452. Mathematical and Maxi7nu7n Error Theory of Aerial 
Photography, June 1, 1941, p. 47 (14). 

453. Computatio7i and Probable Error Theory of Aerial 

Photography, June 1, 1941, p. 11 (14). 

454. Esti77iation of True Target Position by Phototheodolites, 

June 1, 1941 (14). 

455. The Influence of Refractio7i on Positio7i Location, July 
1, 1941, p. 3 (14). 

456. Correclio7is for Two Statio7i Data, Aug. 1, 1941, p. 4 
(14). 

457. Procedure for Theodolite Calculatio7is, Aug. 1, 1941, p. 3 

(14). 

458. Reco77i7ne7ided Methods of Calculatio7i and Theodolite 
Locatio7is, Aug. 1, 1941, p. 4 (14). 

459. Ca7nera Timing Studies, Apr. 18, 1942, p. 3 (14). 

460. Apparent Accuracy of True Height Determinatio77S, 
Sept. 14, 1942, p. 2 (14). 

461. A Co7ifide7ice hiterval for True Height, Sept. 14, 1942, 
p.4 (14). 

462. Effect of Cha7ige of Poiver on Spread of R. F. Readings 

(Preliminary), July 1, 1941, p. 2 (6). 


463. Reproducibility of R. F. Deter7nmatio7is with 12 and 24 

Power; Fixed Targets, Aug. 1, 1941, p. 2 (6). 

464. RCS and Poiver, Aug. 1, 1941, p. 1 (6). 

465. Net Correctio7i to RCS and Power, Aug. 1, 1941, p. 1 

( 6 ). 

466. Co7npariso7i of 12 vs. 24 Poiver on Aerial Courses (Very 
Preliminary), Apr. 18, 1942, p. 5 (6). 

467. The Effect of Power on the Stability of Range Readings 
(Fixed Targets), Apr. 18, 1942, p. 5 (6). 

468. Variation of Height Fmder Perforinaiice with Type of 
Filter Used, Oct. 5, 1942, p. 5 (8). 

469. Ocular Tests and So77ie Case Histories (Prelmiinary Re- 
port on Expeimient I), July 1, 1941, p. 4 (13). 

470. Psycho-physiological Selection Tests and Two Case His- 

tories (Prelhiimary Report on Expermieiits Wl, W3, W4, 
TF5j, July 1, 1941, p. 7 (13). 

471. Screening Tests for Class 11, Nov. 26, 1941, p. 14 (13). 

472. School Perforinance of Classes 7-10, Jan. 29, 1942, p. 23 
(14). 

473. Prediction of Theory and Records Performance from 
Pencil and Paper Tests, Jan. 29, 1942, p. 7 (13). 

474. Psycho77iotor Tests and Height Finder Peiformance, Jan. 

29', 1942, p. 18 (13). 

475. Steadmess Test, Jan. 29, 1942, p. 3 (13). 

476. Ophthal77iograph Tests, Jan. 29, 1942, p. 4 (13). 

477. Pulse Rate, Blood Pressure, Metabolic Rate, Jan. 29, 
1942, p. 7 (13). 

478. Age, Height and Weight, Religious Interest, Rural and 

Urban Birth and Residence, Jan. 29, 1942, p. 3 (13). 

479. Mental Ability, Spatial Relations and Mechanical Com- 
prehension Tests and Log UOE, Jan. 29, 1942, p. 17 (13). 

480. Neurotic Teiidency, Jan. 29, 1942, p. 6 (13). 

481. Psycho-physical Perforiiiance Data, Jan. 29, 1942, p. 21 
(13). 

482. Reliability of Space Eikonoiiieter Settings, Apr. 18, 1942, 

p.2 (13). 

483. Analysis of Repeated Eikonoinetric Tests, Class 12, July 

2, 1942, p. 6 (13). 

484. Analysis of M2 Tramer Data, Eustis II, July 2, 1942, p. 

10 (13). 

485. Visual Acuity and Focus, Eustis II, July 2, 1942, p. 1 (13). 

486. Reliability of hxterpupillary Distance Measure7ne7its, 

July 2, 1942, p. 1 (13). 

487. Visual Acuity Measure77ients, Eustis II, July 2, 1942, p. 
3 (13). 

488. UOE Grades, Class 12 and Previous Classes, July 2, 1942, 

p. 3 (14). 

489. Relation Betwee7i Fixed and Aerial UOE, School Classes 

7-12, July 2, 1942, p. 3 (14). 

490. Reliability of School UOE, July 2, 1942, p. 1 (14) . 

491. Co777parison of School Perfor7na7ice of Class 12 with Test 
III, July 2, 1942, p. 4 (13). 

492. Eikono77ieter Test, Eustis II. A77alysis of Total Score, 
July 2, 1942, p. 4 (13). 

493. Visual Acuity and Test Perfor777ance, Eustis II, July 2, 
1942, p. 7 (13). 

494. Eustis III— Distribution of Stereo Vertical Scores, July 

21, 1942, p. 1 (13). 


RESIRI 


[CTEIT^ 


BIBLIOGRAPHY 


191 


495. Comparative Reliability of Fixed and Aerial Trials, M2 
Trainer, July 21, 1942, p. 2 (13). 

496. M2 Trainer Performance and Time Required to See 
Stereo, Eustis II, July 21, 1942, p. 2 (13). 

497. Eikonometer Sub-tests, Eustis II, July 21, 1942, p. 3 (13). 

498. Summary of Analysis of Vectograph Test, Eustis II, July 

21, 1942, p. 8 (13). 

499. Leaf Room Examination and Height Finder Perform- 
ance, July 21, 1942, p. 2 (13). 

500. Comparison of Old and Neiu Forms of Stereo-Vertical 
Test, July 21, 1942, p. 3 (13). 

501. Correlation of Old and Neiv Stereo-Vertical Test with 

Class 12 Performance, Nov. 25, 1942, p. 3 (13). 

502. Screening Tests, Fort Eustis I (January 1942), July 21, 
1942, p. 23 (13). 

503. Arithmetic and Aircraft Identification Tests. Fort Eustis 
I, July 21, 1942, p. 1 (13). 

504. Relation of Previous Experience to Class Performance, 

Classes 10 and 11, Aug. 25, 1942, p. 1 (13). 

505. Refractive Examination of Class 11— Analysis by Dr. 
Bridgman (Ohio State University), Aug. 25, 1942, p. 6 
(13). 

506. Phoria and Variability of Fixed Target Readings, Aug. 
21, 1942, p. 6 (13). 

507. Instrument Differences and Observer Performance, Aug. 

25, 1942, p. 3 (14). 

508. Aniseikonia and Height Finder Performance of Classes 
8-10, Dec. 11, 1942. 

509. Modification of the M-2 Stereoscopic Trainer, Feb. 12, 

1942, p. 6 (13). 

510. Coincidence vs. Stereo Height Finders, Feb. 12, 1942, 
p. 6 (13). 

511. Mihalyi vs. Stereo Height Finders, Mar. 28, 1942, p. 15 
(3). 

512. Effects of End-Window Stops on Height Finder Perform- 
ance, March 28, 1942, p. 20 (4). 

513. Height Finder Errors Caused by Pentasystem Rotation, 

July 2, 1942, p. 7 (8). 

514. The Interaction between Dependence of Range on 

Eccentric Stops and Internal Adjuster Readings, July 2, 
1942, p. 7 (4). 

515. Height Finder Behavior Analyzed in Terms of Points-of- ' 
View and Reticle Images, July 16, 1942, p. 12 (4). 

516. Location of Stops in the MI Height Finder, July 21, 1942, 

p. 11 (4). 

517. Computation of the Change in Focus of an H.F. Objec- 
tive with Temperature (Revised), Oct. 25, 1942, p. 6 (5). 

518. Change of Focus in the Ml-E Height Finder When 
Changing from Nitrogen to Helium, July 25, 1942, p. 5 
(5). 

519. The Point-of-View from Tivo Circular Stops, July 25, 

1942, p. 11 (4). 

520. The Effect of Interocular Setting Error on Point-of-Vieiv 
Separation, July 25, 1942, p. 4 (4). 

521. Effect of Interocular Setting Changes on Range and RCS 

Readings, Aug. 26, 1942, p. 21 (4). 

522. Effect of Interocular Setting on Consistency, Aug. 27, 
1942, p. 5 (4). 

523. Dependence of Observer Differences on Apj)arent Ob- 
jective Focus, Aug. 27, 1942, p. 2 (8). 


524. Further Ray Tracking Through the Ml-E Objective, 
Oct. 8, 1942, p. 5 (2). 

525. The Dependence of Maximum Yard Error on Range of 
No Parallax, Oct. 8, 1942, p. 9. (4). 

526. Perspective Error in Ortho-Psuedo-Stereoscopic Range- 
finders (Revised), Jan. 18, 1943, p. 3 (4). 

527. Spherical Aberration in the Ml-E Height Finder Objec- 

tive and its Effect on Perspective Error, Oct. 8, 1942, 
p.6 (4). 

528. A Different Method of Obtaining Eye Focus, Apr. 18, 

1942, p. 10 (12). 

529. Report on Comparison of Visual Acuity Measurements 
by Mass. Vision Test and Project-O-Chart, Aug. 31, 1942, 
p. 2 (13). 

530. A Study of Examiner Differences, Sept. 14, 1942, p. 6 
(13). 

531. Tracking Errors in Height Finding, Sept. 22, 1942, p. 6 

( 10 ). 

532. A Theoretical Study of the Effectiveness of an A. A. Bat- 
tery, Mar. 31, 1942, p. 30 (1). 

533. Scoring Height Finder Performance, Mar. 31, 1942, 

p. 6 (14). 

534. One-dimensional Prediction Problems and the Theory 
of Rating Locators for Predicting Poxcer, Mar. 31, 1942, 

p. 6. 

535. Computations for Rating of One-dimensional Locators 
for Predicting Power, Mar. 31, 1942, p. 12. 

536. The Way in which Probability of Hitting with a Time- 
Fused Projectile Varies with Muzzle Velocity, Mar. 31, 
1942, p. 18. 

537. Formulas for the Probability of an Effective Hit on Air- 
craft by A. A. Fire, Aug. 21, 1942, p. 14. 

538. Accuracy of the Approximations Involved in the Fac- 
torization of the Probability of Effective Hitting, and in 
the Relation of the Preburst Factor to the Trivariance, 
June 19, 1942, p. 5. 

539. The Application of Vector, Dyadic and Pluri-xiariance 
Technique to the Burst Problem, June 19, 1942, p. 5. 

540. The Computational Applicatioxi of Pturwariance to the 
Discussion of Hitting Problems, June! 19, 1942, p. 18. 

541. Elementary Exposition of Dead-Time Error in a Con- 
tmuously Pointing System, Aug. 31, 1942, p. 10. 

542. Scoring Methods for Antiaircraft Fire Control Equipment 
and Applications to Fort Monroe Director Tests, Oct. 20, 
1942, p. 34. 

543. Scoring Locator Performaxice, Jan. 26,* 1943, p. 42. 

544. The Employment of Model Predictors in Analyzing Di- 
rector Performance, Jan. 28, 1943, p. 35. 

TANK ARMAMENT RESEARCH COMMITTEE 

545. Trials of Barr and Stroud Rangefinder No. 2, Apr. 24, 

1944, p. 11 (15). 

546. Trials with Australian Rangefinding Sight, Parts 1-3 
(non-firing trials), Dec. 23, 1943, p. 27 (15). 

547. Firing Trials with Australian Rangefinding Sight, Dec. 
29, 1943, p. 8 (15). 

TUFTS COLLEGE 

548. Fatigue Tests. The Effects of Sounds on Accuracy of 

Azimuth Tracking (Report 1), OEMsr-581, Sept. 18, 1942, 
p. 34 (9). Div. 7-220.18-M2 


JESTRICTED 


192 


BIBLIOGRAPHY 


549. Fatigue Tests. Continuous iii /2 Hours Tracking Fatigue 
Test. (Report 2), OEMsr-581, Feb. 1943, p. 7 (9), 

Div. 7-220.16-M2 

550. Fatigue Tests. Three-Day Test of Fatigue Effects Under 
Conditions of Long Hours on Duty, Lhnited Sleep. 
(Report 3), OEMsr-581, Feb. 1943, p. 18 (9). 

Div. 7-220.16-M3 

551. Fatigue Tests. Effect of Sleep Deprivation Upon Per- 
formance (Report 4), OEMsr-581, Feb. 1943, p. 37 (9). 

Div. 7-220.16-M4 

552. Fatigue Tests. Motivation in the Performance of Routine 

Military Tasks (Report 5), OEMsr-581, Feb, 1943, p. 18 

(9). Div. 7-220.16-M5 

553. Fatigue Tests. Effect of a Thirty-mile Hike on Stereo 

Rangmg, Tracking and Other Tasks (Report 6), OEMsr- 
581, Feb. 1943, p. 40 (9). Div. 7-220.16-M6 

554. Fatigue Tests. The Effect of Short-Period Exercise on 

Stereo Ranging. (Report 7), OEMsr-581, Feb, 1943, p. 10 
(9). Div. 7-220.16-M7 

555. Fatigue Tests. The Effect of Diopter Settings on Stereo 

Performance (Report 8), OEMsr-581, Apr. 24, 1943, 
p. 19 (12). Div. 7-220.16-M9 

556. Report, Feb. 1942 (4) (10). 

557. The Tufts Trainer, Apr. 14, 1942, p. 31 (13) (14). 

Div. 7-220.211 -Ml 

558. Constant Error and Variability in the Tufts Trainer as 
Influenced by Techniques of Bracketing on a Stationary 
Target, Apr.' 16, 1942, p. 8 (12). Div. 7-220.211 -M2 

559. Bell Pacing on a Moving Target, Apr. 18, 1942, p. 34 (9). 

Div. 7-220.18-Ml 

560. The Target Position Effect on Constant Error in The 

Tufts Trainer and in the Navy Trainer, Mark II, Model 
2, Apr. 20, 1942, p. 12 (10). Div. 7-220.19-Ml 

561. The Effect of the Inter-Eyepiece Distance Setting in the 
Navy and Tufts Trainers on Constant Error, And, An 
Apparatus for the Accurate and Reliable Measurement 
of Inter-Eyepiece Distance, Apr. 20, 1942, p. 9 (4). 

Div. 7-220.19-M2 

562. Reticle Design. The Circle Reticle, Apr. 22, 1942, p. 5 

(16). ^ Div.7-210.31-Ml 

563. Knowledge of Results Training in Ranging on a Moving 
Target, Apr. 22, 1942, p. 8 (9) (14). Div. 7-220.14-Ml 

564. The Test-Retest Reliabilities of the Bott Test of Stere- 
opsis, the Naxiy Mark II Stereo-Trainer and the Tufts 
Stereo-Trainer, Leonard Carmichael, Bertram Wellman, 
and others, Apr. 24, 1942, p. 12 (12). Div. 7-220.15-M2 

565. Intercorrelations of Scores on the Bott Test of Stereopsis, 

the Navy Mark II Stereo-Trainer and the Tufts Stereo- 
Trainer, l.eonard Carmichael, Bertram Wellman, and 
others, Apr. 24, 1942, p. 29 (13). Div. 7-220.15-Ml 

566. The Test-Retest Reliability Coefficients of the Tufts 

Stereo-Trainer, the Navy Stereo-Trainer Mark II and the 
Bott Test of Stereo Acuity, June 18, 1942, p. 16 (13). 

Div. 7-220.15-M4 


567. Comparison of Men and Women Subjects on the Tufts 
Stereo-Trainer, the Navy Stereo-Trainer Mark II and the 
Bott Test of Stereo Acuity, June 18, 1942, p. 10 (9) (13). 

Div.7-220.15-M5 

568. The Correlation Between the Keystone and Bott Tests 

of Stereopsis and Influence of Size of Test Stimuli in 
Judgments of Stereopsis, June 18, 1942, p. 8 (13). 

Div. 7-220.15-M6 

569. Intercorrelations Between Scores on the Tufts Stereo- 
Trainer, the Navy Stereo-Trainer, the Bott Test of 
Acuity, the Vectographic Pursuit Apparatus, the Wulfeck 
Group Test of Stereo Acuity and Two Tests of General 
Intelligence, June 18, 1942, p. 30 (13). Div. 7-220.15-M3 

570. High Impedance Direct-Reading Vacuum Tube Volt- 
meter for DC, July, 1942, p. 14. Div. 7-220.21 1-M3 

571. The Effect of a Tivo-Week Layoff After Training on 

Stereo Performance, OEMsr-66, July 15, 1942, p. 4 (9). 

Div.7-220.17-M2 

572. Fatigue and Motivation, 1943, p. 12 (9). 

Div.7-220.16-M12 

573. The Performance of Trained Subjects on a Complex 
Task of Four Hours Duration, July 24, 1943, p. 8 (9). 

Div. 7-220.16-Mll 

574. The Relationship Between Eyepiece Diopter Settings and 

Stereo Range Settings, Aug. 3, 1943, p. 11 (12). 

Div. 7-210.33-M2 

575. Summary Report of Research and Development Work 

from August 1, 1942 to July 1, 1943, June 30, 1943, 
p. 10 (10). Div.7-220.16-M10 

576. Experiments with British Seamen at Fort Monroe and 
Providence, October 8-17, 1941, October 1941, p. 20 (13). 

Div. 7-220.1 1-M2 

577. Survey of Experiments Performed at Tufts College, 
June 22, 1943, p. 22 (10). 

VARIOUS (SUPER RANGE FINDER) 

578. CotJiments on Branch Memorandum No. 5, p. 35 (17). 

579. Letter from T. C. Fry to Col. W. R. Gerhardt dated 
Oct. 7, 1943 (15). 

580. The Application of the Wulfeck Group Test of Stereo 

Acuity at Randolph and Kelly Fields, May 25 to 30, 1942, 
Samuel W. Fernberger, May 1942, p. 14 (13). 

Div. 7-220.12-M5 

581. A Proposed Group Test of Stereo Acuity, 1942, p. 11 

(13). Div. 7-220.12-M23 

582. Second Report re: Validatioti of Tests Against the Navy 

Scorhig System, Samuel W. Fernberger, Oct. 19, 1942, 
p. 5 (13). Div. 7-220.15-M9 

583. Stereo Acuity Testing on Aviation Pilot Candidates at 
the Philadelphia Navy Yard. 1942, p. 9 (13). 

Div. 7-220.12-M3 

584. The Validation of Tests Against the Navy Marking 
Systefn, Samuel W. Fernberger, Sept. 22, 1942, p. 5 (13). 

Div. 7-220.15-M8 


The text of this technical monograph was closed on February 22, 1945. For the sake of 
completeness, the following section includes several titles dealing with rangefinders and their 
operation which were received from that date until October 1, 1945. However, summaries of 
these titles do not appear in the text. 

DIVISION 7, NDRC 

586. Studies in Lead Tracking xvith Machine Gun Sights, 

585. Influence of Visual Magnification on Accuracy of Track- Division 7, RS 92, OEMsr-453, The Foxboro Company, 

ing, RS 90, Apr. 1945, p. 3. Ji>«e 1945, p. 6. Div. 7-220.32-M2 


RESTRl 


BIBLIOGRAPHY 


193 


587. Studies of the Design of Illuminated Reticles for Stereo- 
scopic Range and Height Finders, RS 93, July 1945, p. 3. 

588. On a Modified Internal Adjuster System for Stereoscopic 
Rangefinders, RS 94, Sept. 1945, p. 3. 

589. On the Relation of Atmospheric “Boil” to Magnification 
and Base Length of Stereoscopic Ranging Instruments, 
RS 95, Sept. 1945, p. 3. 

590. Factors Influencmg the Magnitude of Range Errors in 
Free Space and in Telescopic Vision, Oct. 1945, p. 7. 
(Included in Ref. 283.) 

ABERDEEN PROVING GROUND 

591. Rangefinder, 70 cm base. Stereoscopic (Model IV /a) 
German (FMFC 367), Jan. 16, 1945, p. 5. 

592. Rangefinder, 1.5 meter base. Model 96, Japanese (FMFC 
372), Feb. 12, 1945, p. 6. 

APPLIED PSYCHOLOGY PANEL, NDRC 

593. Generality of Tracking Training, Dec. 28, 1944, p. 8. 

594. A Study of Errors of Prediction Resulting from Azimuth 
Tracking Errors in the Director M7, Dec. 28, 1944, p. 13. 

595. Manual for the histallation and Adjustment of the 
Multiple Projection Eikonometer, Oct. 10, 1944, p. 73. 

ARMY ORDNANCE RESEARCH GROUP [AORG] 

596. The Accuracy of Range Finders in H.E. Shoots, Jan. 23, 
1945, p. 15. 

ADMIRALTY RESEARCH LABORATORY 

597. German Rayigefinder Trainer for Use with Em. 4 m. 
R.H-34 and 36 Rangefinders, Jan. 24, 1945, p. 4. 

598. The Removal of Elevation Error from the Army Height- 
finder No. 3. Type U.B. 7 by Circumferential Air Stirring, 
Jan. 25, 1945, p. 12. 

BROWN UNIVERSITY 

599. Summary of Research on the Design of Illuminated 

Reticles, Clarence H. Graham, Lorrin A. Riggs, and 
others, OEMsr-1059, June 16, 1945, p. 17. (Included in 
Ref. 587.) Div.7-210.31-M10 

600. A Modification of the Internal Adjuster System of the 

Stereoscopic Range Finder Mark 65, With Comparative 
Data on the Effects of Temperature on the Modified and 
the Standard Instruments, Lorrin A. Riggs, C. G. Mueller, 
and F. A. Mote, OEMsr-1059, July 21, 1945, p. 9. (In- 
cluded in Ref. 588). Div. 7-210.112-M5 

601. Photographic Measurements of Atmospheric Boil, with 
Some Preliminary Theoretical Considerations of the 
Relation of Boil to Range Finder Magnification and 
Base Length, Lorrin A. Riggs, C. G. Mueller, and others, 
OEMsr-1059, Aug. 6, 1945, p. 13. (Included in Ref. 589.) 

Div. 7-210-M6 

FOXBORO COMPANY 

602. Influence of Visual Magnification on Accuracy of Track- 

ing (Memorandum 14), OEMsr-453, Apr. 5, 1945, p. 6. 
(Included in Ref. 585.) Div. 7-220.312-M2 

603. Lead Tracking Errors ivith Optical Ring Sights versus 
Sights with Conventional Reticles, Feh. 19, 1945, p. 21. 
(Included in Ref. 586.) 

604. Influence of Radial Reticle Lines on Accuracy of Lead 
Tracking. Dec. 23, 1944, p. 11. (Included in Ref. 586.) 

605. Influence of Reticle Lines and Center Dot on Tracking 
Accuracy, May 19, 1945, p. 23. (Included in Ref. 586.) 

606. Preliminary Investigation of Tandem Tracking (Memo- 
randum 24), OEMsr-453, June 28, 1945, p. 13. 

Div. 7-220.31 -M2 


607. Studies in Aided Tracking, Aug. 31, 1945, p. 43. 

608. A Simple Scoresby (Rolling Platform) Equipment for 
Testing Tracking Apparatus, Aug. 31, 1945, p. 9. 

GALILEO COMPANY 

609. Device for Protecting the External Optical Surfaces of 
Optical Instruments for^ the observation or Measurement 
of Obfuscation Due to Smoke, Dust, etc. (Italian Patent 
13141), June 22, 1942, p. 3. 

HARVARD UNIVERSITY 

610. Factors Influencing the Magnitude of Range Errors in 
Free Space and in Telescopic Vision, Alfred H. Holway, 
Dorothea A. Jameson, and others, OEMsr-555, Aug. 10, 
1945, p. 314. (Included in Ref. 590.) Div. 7-210.11-M7 

611. Final Report No. 27 on Contract NDCrc-186, Volume I, 

Summary, NDCrc-186, Princeton University, Jan. 31, 
1943. Div.7-201-Ml 

612. Final Report on Contract NDCrc-186, Volume II, Report 
Nos. 1-16, NDCrc-186, Princeton University, Jan. 31, 1943. 

Div. 7-201 -M2 

613. Final Report on Contract NDCrc-186, Volume III, Re- 

port Nos. 17-26, NDCrc-186, Princeton University, Jan. 
31, 1943. Div.7-201-M3 

614. Final Report on Contract NDCrc-186, Volume IV, Re- 

ports of Progress and Proposed Programs, NDCrc-186, 
Princeton University, Jan. 31, 1943. Div. 7-201-M4 

615. Final Report on Contract NDCrc-186, Volume V, Ster- 

eoscopic Testing Center Progress Reports, NDCrc-186, 
Princeton University, Jan. 31, 1943. Div. 7-201-M5 

616. Final Report on Contract NDCrc-186, Volume VI, 

Height Finder Studies Nos. O-VIII, NDCrc-186, Prince- 
ton University, Jan. 31, 1943. Div. 7-201-M6 

617. Final Report on Contract NDCrc-186, Volume VII, 

Height Finder Studies Nos. IX-XVI, NDCrc-186, Prince- 
ton University, Jan. 31, 1943. Div. 7-201-M7 

618. Final Report on Contract NDCrc-186, Volume VIII, 

Director Studies, NDCrc-186, Princeton University, Jan. 
31, 1943. Div.7-201-M8 

619. Range and Height Finder Problems. Comments on 

Branch Memorandum 5, Apr. 1943. Div. 7-210-M3 

620. Final Report on Contract OEMsr-1016 (Part I), Experi- 
mental Range Finder Type I, and a Study of Aided Rang- 
ing, Harry G. Ott, OEMsr-1016, Bausch and Lomb 
Optical Co., May 30, 1945. (Part II) Status as of January 
31, 1945 of the Development of a Range Finder in 
Accordance with Proposal No. 4 Referred to in the 
Minutes of Meeting of Steering Committee for Project 58 
on September 15, 1943, O. H. Wolferts, OEMsr-1016, 
Bausch and Lomb Optical Co., Jan. 31, 1945. 

Div. 7-210-M5 

Testing of Personnel. Progress Report of Project No. 10, 
Clarence H. Graham, Lorrin A. Riggs, and others, 
OEMsr-66, Brown University, Feb. 9, 1942. 

Div. 7-220.1 -M2 

Visual Acuities in Telescopic Vision, Alfred H. Holway, 
Dorothea A. Jameson, and others, OEMsr-555, Harvard 
University, Dec. 28, 1945. Div. 7-220.12-M22 

Tests of Stereoscopic Vision for the Selectioji of Range 
Finder Operators [Tests on Tufts University ROTC 
Students], Harvard University, June 26, 1942. 

Div.7-220.14-M8 


621. 

622. 

623. 


RESTEICTEIT 







INDEX 


The subject indexes of all S TR volumes are combined in a master index printed in a separate volume. For access 
to the index volume consult the Army or Xavy Agency listed on the reverse of the half-title page. 


Abbe internal adjuster system, 62 
Aberdeen Proving Ground, 20, 177 
Accuracy of ranging; see Ranging ac- 
curacy 

Admiralty Research Laboratory, 35, 46, 
64 ' 

Advanced Fire Control School, 102 
Aerial perspective, effect on ranging, 94 
Aerial photography for range finder 
scoring, 133 

Aerial target, position determination, 
133 

Aided tracking, handwheel speeds with, 
88, 89, 99 

Air stirring in range finders, 36 
American Cyanamid Company, 31 
American Gas Association Testing Lab- 
oratories, 2, 41, 45 
Angular tracking errors, 84 
Aniseikonia, 73, 82, 116 
Aperture oscillator, 48 
Applied Mathematics Panel, XDRC, 55 
Applied Psychology Panel, XDRC, 1, 2, 
125 

Armored vehicle, use of short base 
range finder, 151 

Army general classification test, 115, 
118 

Army Range Finder Xo. 3 type U. B. 
7; 37 

Army reticle, 161, 164, 165 
Artificial haze, 95 
Aubert-Foerster effect, 11 
Australian Range Finding Sight, 156 
Axialometer, 32 

Barr and Stroud range finders, 37, 151, 
155 

Base length, effect on range finder ac- 
curacy, 55 

Bausch and Lomb Duplex P. D. Gauge, 
31, 114 

Bausch and Lomb Optical Co.; color 
differentiation, effect on ranging, 
9 

flicker, effect on ranging, 9 
perspective error, 32 
range errors due to helium charging, 
48 

target position, effect on ranging, 9 
visual range estimation tests, 153 
Bausch and Lomb ortho-rater, 116, 145 
Beam-splitter, polarizing optical, 177 
Bennett mechanical comprehension test, 
112 

Benzedrine sulphate, effect on ranging 
efficiency, 78 

Bernreuter personality inventory, 126 

Binocular fusion, 69 

Binocular fusion reaction time test, 128 


Binocular vergence, 69 
Binoculars altered for use as range 
finders, 149 

Bott test of stereopsis, 117 
Bracketing methods, 99-103 
British; Barr and Stroud range finder 
tests, 152 

circumferential air stirring, 46 
comparison of British and American 
range finders, 17-18 
computing instruments for optical 
range finder, 145 

elevation errors in range finders, 35, 
46 

FQ25 Range Finder, 17 
height of image error, 103 
Infantry Range Finder Xo. 12; 153 
Levallois Range Finder, 151 
range finder for tank gunnery, 153 
rhodium coatings for range finder 
windows, 175 

selection tests for range finder oper- 
ators, 111 

temperature control of range finders, 
46 

UB7 Range Finder, 17 
visual range estimation tests, 152 
Broken contact method of ranging, 135 
Brown Stereoscopic Trainer, 77, 138 
Brown University; effect of loud sounds 
on stereoscopic ranging, 77 
emotional stability tests, 126 
height finder sunshade, 47 
internal adjustment settings, 61 
measurement of interpupillary dis- 
tance, 114 

personal inventory test, 131 
reticle design, 102, 161 
target and reticle separation, 83 
Bureau of Xaval Ordnance, 31 
Bureau of Standards; see Xational Bu- 
reau of Standards 

California Institute of Technology 
(CIT) range finder tests, 5 
Cameras for checking target position, 
133 

Camp Wallace testing center, 109 
Casella visibility meter, 152 
Chromastereopsis, 97 
Chromatic dispersion of the human eye, 
97 

Circulating air in range finders, 36, 46 
CIT optical range finding systems and 
tests, 5-8 

“Clipping” errors, 32 
Coincidence acuity, 11 
Coincidence range finders, 5, 17, 35 
Coincidence strip range finders, 5 
Cold weather height finder tests, 47 


Consis,tency error, definition, 134 
Continuous contact method of ranging, 
135 

Contrast meter for tracking telescope, 96 
Curve B effect, 9-1-95 
Cyclo tests, 123 
Cyclophoria, 70, 83 

Dartmouth Eye Institute, 73 
Dartmouth Tilting Board, 73 
Dearborn-Johnson tests, 109, 120 
Depth perception tests, 120 
DeYoe slide rule, 145 
Differential estimation of range, 154 
Diopter settings, 101 
Dioptometer, 48 

Division 7 XDRC, range finder develop- 
ment history, 1-4 

Eastman Kodak Co.; ortho-pseudo range 
finders, 18, 151, 177 
penta-reflector errors, 50, 52 
short base range finders, 149-151 
Eastman Trainer, tests with; effect of 
disuse on ranging efficiency, 75 
effectiveness in training height finder 
operators, 141 
range finder fields, 157 
ranging on a diving target, 169 
studies on a diving target, 169 
studies on relation of reticle to target, 
83 

Eikonometer tests, 107-112, 116, 123-125 
Electrical jacket for range finders, 41, 45 
Electric shock, effect on stereo-acuity, 
127 

Electroencephalographic technique, 126 
Elevation error, control, 35-50 
Elevation - tracking - error compensation 
knob, 6 

Emotional stability tests, 126-132 
End-window stops, 25 
Eyepieces, focussing, 101 

False fusion in reticle patterns, 170 
Fire control linkage with range finder, 
155 

Focal change due to helium charging, 
48 

Focussing techniques, telescopic eye- 
pieces, 101 

Fogging of optical instruments, 41, 47 
Foot ranging controls for tracking, 160 
Fore and aft marks on reticles, 164 
Fort Eustis testing center, 109 
Fort Lauderdale, Florida, 2 
Fort Monroe, Princeton Laboratory; see 
Princeton Laboratory, Fort 
Monroe 


RESTRICTED 


195 


196 


INDEX 


I'oxboro Company; hanchvheel track- 
ing, 84 

sex differences in tracking, 78 
simultaneous tracking and stadia 
ranging, 158 

stadia ranging reticles, 173 
F. Q. 25 Range Finder, 35, 46 
Frankford Arsenal; cold weather tests 
on height finders, 47 
comparative study of field types, 9 
leveling and aligning Ml Height 
Finder, 63 

measurement of focal differences, 48 
range estimation studies, 153 
reduction of perspective error, 28 
reticle design, 161 
“Super Range Finder”, 178 
Full-line reticle, 161 

General classification test, 115, 118 
General Electric Company, 41 
German R-40 Range Finder, 20, 176 
German R-40 reticle, 167, 168 
Goerz Range Finder No. 8; 37 
Gray circle reticle, 169 
Gun directors, M4 and M7; 91 
Gun motor carriage, range finder for, 
156 

Hand and foot technique for simul- 
taneous tracking and ranging, 
159 

Hand control for simultaneous tracking 
and ranging, 159 
Handwheel tracking, 84, 87 
Haploscope, 70 

Harvard Fatigue Laboratory; binocular 
fusion and vergence, 69 
effect of drugs on ranging, 78 
hyperventilation and exercise, effects 
on ranging, 72 

interocular devices, self locking, 32 
interocular measurements, 29, 113 
reticle and target position, 83 
stereoscopic acuity measuring appa- 
ratus, 138 

visual range observations, 10-16 
Harvard Psycho-Educational Clinic, 117, 
118 

Haze production, artificial, 95 
Haze, effect on ranging, 95 
Height error due to tracking error, 84 
Height Finder School at Camp Davis, 
N.C., 59, 109 

Height finders; see also Ml Height 
Finder, M2 Height Finder, Range 
Finders, Ranging accuracy 
as a spotting instrument, 103 
calibration, 59 

charging with helium, 38, 44, 45, 48 
cold weather tests, 47 
electrically heated jacket, 41, 47 
eye focus, 101 
fogging, 47 
mirrors, 50 
penta-reflectors, 51 
recommendations, 40, 48, 55, 79 
temperature effects, 41, 43 


window stops, 28 

Height of image adjustment, 102, 171 
Helium charging of range finders, 38, 
44-45 

Helium purity indicator, 46 
Howard-Dolman peg test. 111 
Howe Laboratory of Ophthalmology; 
effect of disuse on ranging 
efficiency, 75 

haze effects on ranging, 95 
interval between determining and 
signaling range, 70 
make - and - break versus continuous 
tracking, 99 

ranging on a simulated high speed 
diving airplane, 138, 140 
relation of reticle to target position, 
82 

stereoscopic ranging on different type 
targets, 101 

Hyperventilation effect on ranging per- 
formance, 72, 80 

Infantry range finders, 151-152, 157 
Instruction Manual for Gunnery for 
the Armored Force Schools, 153 
Interaxial distance (lAD), 31, 113 
Interaxialometer, 113 
Internal adjuster target, 61 
Interocular settings, 29-31, 113-115 
Interpupillary distance (IPD), 29, 113 
Interpupillometer (NDRC model), 34, 
114 

Iowa State College, 92 

Iowa State University, 78 

IPD gauge (Harvard), 113 

IPD gauge (Shuron), 30-31, 98, 113-115 

Just noticeable difference (j.n.d.), 13 

Keuffel and Esser range finders, 154 
Keystone test, 116 

Leaf room test, 123 

Levallois Stereoscopic Range Finder, 
151 

Le^el collimator, 64 
Likert-Quasha Minnesota Paper Form 
Board, 112 
Listing’s law, 70 
Luckish-Moss Illuminator, 108 

Ml Height Finder; see also Height find- 
ers, Range finder design and per- 
formance, Ranging accuracy — 
Instrumental factors. Ranging 
accuracy — Psycho-physiological 
factors 

comparison with German R-40 range 
finder, 20 

comparison with Kodak ortho-pseudo 
type, 17 

comparison with “Mickey” (radar), 21 
effectiveness as a training instrument, 
140 

magnitude of errors, 134 
Ml Range Finder; see Ml Height 
Finder 


Ml Stereoscopic Height and Range 
Finders; see Ml Height Finder 
M2 Height Finder; charging with heli- 
um, 43 

electrically heated covers, 4-7 
elimination of thermal errors, 43 
magnification, 54 
penta-reflector errors. 51 
recommendations, 48 
M2 Range Finder; see M2 Height 
Finder 

M2 Trainer, 82, 116, 136, 140 
M2 Trainer tests, 108-112, 121, 125 
M4 Gun Director, 91 
M4 Trainer, 139 

M6 Stereoscopic Trainer, 9, 139, 161 
M7 Gun Director, 91 
M7 Range Finder, 154 
M9 Range Finder, 156, 158 
MIO Range Finder, 151 
M61 Range Finder, 158 
M62 Range Finder, 158 
Manual for charging height finders in 
the field, 43 

Mark 1 range finding sight, 149 
Mark 2 Trainer, 29, 30, 76, 77, 101 
Mark II Trainer, 70, 82 
Mark 4 Trainer, 158; see also Eastman 
trainer 

Mark 37 Stereoscopic Range Finder, 56 
Mark 40 Range Finder, 9 
Mark 42 Range Finder, 32, 60, 75, 99 
Mark 45 Stereoscopic Range Finder, 56 
Mark 46 Stereoscopic Range Finder, 56 
Mark 52 Stereoscopic Range Finder, 56 
Mark 58 Range Finder, 32, 51 
Massachusetts Vision Test Kit Modified, 
108, 116 

Mechanical ability test, 118 
Metrazol, effect on ranging efficiency, 78 
“Mickey” (radar), 21 
Mihalyi instrument, 5 
Monofocle, 48 

Multiple projection Eikonometer, 111, 
123 

National Bureau of Standards; kinds of 
errors in range and height 
finders, 65 

penta-reflector errors, 51 
perspective errors, 25 
thermal errors, 43 

Naval training schools at Fort Lauder- 
dale, 96 

Navy Diamond reticle, 161, 164-168, 170 
Navy Line reticle, 164-171 
Navy Mark 42 Range Finder, 59 
Navy Mark 58 Range F'inder, 154 
Navy Open Diamond reticle, 164, 170 
Navy Post reticle, 161 
Navy Solid Diamond reticle, 164, 167- 
168, 170 

NRC neurotic inventory test, 128, 129 
NRC personality inventory, 126, 128, 
130 

NRC troublemaking inventory, 128 
Ocular separation template, 33 


RESTRICl'ED 


INDEX 


197 


Ohio State Contrast Meter, 90 
Ohio State University; chromatic dis- 
persion of human eye, 97 
cyclophoria studies, 70 
haze effects on ranging, 95 
intermittent visibility of low con- 
trast targets, 98 

maintaining contact on a moving 
target, 99 

sex differences in ranging, 77 
spotting with a height finder, 103 
Oil contamination from ordinary heli- 
um, (range finders), 46 
Oliver helium charging method, 46 
Opaque reticle patterns, 172 
Operators for range finders; see Range 
finder operator. Range finder op- 
erator selection tests 
Ophthalmoeikonometer, 123 
Ophthalmograph test, 115 
Optical bars; comparison of invar and 
steel, 49 
mounting, 52 
thermal effects, 42, 49 
Optical range finders; see Range finders 
Optical ranging, principles, 6 
Optical systems for range finders; see 
Range finder optical systems, new 
Ortho-pseudo range finder, 5, 18-20, 28, 
151, 177 

Ortho-rater, 112, 116, 145 
Otis intelligence test, 129 

Parallactic errors, 32 
Parallax in range finders due to helium 
charging, 48 

Penta-reflector errors, 41, 50-53 
Penta-reflectors of quartz and copper, 
50-52 

Penta-system rotation, 63 
Perkins and Elmer Range Finder, 154 
Personal inventory test, 130 
Perspective error, 23, 25-34 
causes, 25 

control by end-window stops, 25 
control by interocular settings, 30 
effect of interpupillary adjustment, 
29 

interaxial distance measurement, 31 
recommendations, 33 
Perspective parallax, 32 
Photogrammetry, 133 
Phototheodolites, 133, 137 
Polarizing beam-splitter, 177 
Polaroid Corp. short base range finder, 
151-154 

Princeton Laboratory, Fort Monroe; 
aerial perspective and weather 
factors, 94 

contact methods, 101 
elimination of thermal errors, 43 
errors due to penta-system rotation, 
63 

field studies of different range finders, 
17-21 

focussing of telescopic eyepieces, 101 
interpupillary adjustment, 29 
Ml Stereoscopic Height Finder per- 


formance, 134 

perspective error control, 25 
reticle and target position studies, 83 
selection tests for operators, 107-132 
thermal effects in Stereoscopic range 
finders, 38 

true target position, 133 
Projectiles, ranging for, 158 
Projection eikonometer test, 125 
Psychomotor tests, 112 

Quartz height finder mirrors, 50 

R 40 Range Finder, 65 
Radar ranging; comparison with Ml 
Height Finder, 20 

comparison with optical range finding, 
21, 137 

Range correction computer, 144 
Range Corrector Settings; see RCS 
settings 

Range estimation without instruments, 
10-14, 153-155 

Range finder design and performance; 
see also Ranging accuracy — 
Instrumental factors. Ranging 
accuracy — Psycho-physiological 
factors 

aided tracking devices, 99 
armored vehicles use, 153-157 
calibration, 57-62 
end window stops, 25 
recommendations, 25-29, 33, 34, 40-44, 
55, 126 

reticle design, 161-174 
rhodium coated windows, 175 
suggested optimum specifications, 175 
“Super Range Finder”, 178 
temperature control 
air circulation, 36, 46 
electrically heated jacket, 41 
helium charging, 38, 44-45 
sunshades, 42 
temperature tubes, 36 
variable diaphragms, 25 
Range finder operator; accuracy of per- 
formance, 133 
aniseikonia, 73 
cyclophoria, 70 
fatigue, 70 
hyperventilation, 72 
improvement with training and selec- 
tion, 137 

individual differences, 70 
learning curve, 144 
measurement of interocular distance, 
112 

minimum standards, 107 
motivation, 71, 74 
selection tests, 107-132 
standardization scores, 108 
training aids, 144-145 
training manual, 74, 142-144 
training program, 133, 140 
training schools, 137 
Range finder operator selection tests; 
Army general classification test, 
115, 118 


Bausch and Lomb ortho-rater stereo- 
scopic test, 125 

Bennett mechanical comprehension 
test, 112 

Bernreuter personality inventory, 126 
binocular fusion reaction time test, 
128 

Bott test of stereopsis, 117 
British selection tests, 111 
cyclo tests, 123 

Dearborn-Johnson tests, 109, 120, 125 
depth perception tests, 120 
dynamic tests, 118 

eikonometer tests, 107-112, 116, 122- 

125 

electroencephalographic technique, 

126 

emotional stability tests, 126 
general classification test, 115, 118 
Howard-Dolman peg test. 111, 116 
Keystone test, 116, 125 
Likert-Quasha Minnesota paper form 
board, 112 

M2 Trainer tests, 70, 108-112, 116, 
121, 125 

manual for administration of tests, 
108 

Modified Massachusetts vision test 
kit, 108, 116 

mechanical ability test, 118 
multiple projection eikonometer, 110, 
123 

NRC neurotic inventory, 128, 129 
NRC personal inventory, 126, 128, 130 
NRC troublemaking inventory, 128 
ophthalmo-eikonometer, 123 
opthalmograph test, 115 
ortho-rater stereoscopic test, 125 
Otis intelligence test, 129 
personal inventory test, 130-131 
projection eikonometer test, 125 
psycho motor tests, 112 
Rorshack ink blot test, 127 
space eikonometer, 73, 123 
static tests, 118 
steadiness test, 112 
stereovertical test, 108, 109, 123 
two-hand coordination test, 129 
validity of, 108 

vectograph pursuit test, 109, 117, 119, 
125 

Verhoeff size-confusion tests, 118 
visual acuity test, 115 
Willoughby' test, 128, 129 
Wulfeck vectograph test, 117-118, 125 
Wunderlich-Hovland personnel test, 
112, 128, 129 

Range finder optical systems; coinci- 
dence strips, 5 

invert coincidence ortho-motion, 10 
invert coincidence pseudo-motion, 10 
invert ortho-pseudo, 10 
ortho-pseudo, 5, 10, 18-20, 28, 151, 
177 

simple coincidence with flickered 
images, 5 

simple coincidence with colored fil- 
ters, 5 


^ REST1<sM:TKD 


198 


INDEX 


simple full field coincidence, 5, 10 
split field coincidence type, 17, 35 
stereo strips, 5 

superimposed coincidence, 10, 178 
Range finder slide rule, 145 
Range finder types; Army Finder No. 3; 
37 

Australian Range Finding Sight, 156 
Barr and Stroud Range Finder Mark 
VI, 154 

Barr and Stroud Range Finder No. 2; 
152 

Barr and Stroud Range Finder No. 
10; 37 

Barr and Stroud Range Finder No. 
12; 151 

Eastman Kodak short base range 
finder, 149-151 
for gun motor carriage, 156 
for tank gunnery, 153 
F. Q. 25 Range ifinder, 17, 35, 37, 46 
German R-40 Range Finder, 20, 65, 
176 

Goerz Range Finder No. 8; 37 
Infantry range finders, 152, 156, 157 
Keuffel and Esser range finders, 154 
Levallois Range Finder, 151 
Ml Range Finder, 58, 64 
M2 Range Finder, 58 
M7 Range Finder, 154 
M9 Range Finder, 156, 158 
MIO Range Finder, 151 
M61 Range Finder, 158 
M62 Range Finder, 158 
Mark 1 Range Finding Sight, 149 
Mark 37 Stereoscopic Range Finder, 
56 

Mark 40 Range Finder, 9 
Mark 42 Range Finder, 32, 59, 75, 
99 

Mark 45 Stereoscopic Range Finder, 
56 

Mark 46 Stereoscopic Range Finder, 
56 

Mark 52 Stereoscopic Range Finder, 
56 

Mark 58 Range Finder, 32, 51 
Perkins and Elmer Range Finder, 154 
Polaroid Corp. short base range find- 
ers, 151-154 

proposed new instruments, 175 
R-40 Range Finder, 65 
Riggs reticle, 161 
short base, 149 
“Super Range Finder”, 178 
superimposed type, 178 
T16E1 Range Finder, 158 
T25 Range Finder, 151 
T26 Range Finder, 151 
U.B.7 Range Finder, 17 
U.D.4 Range Finder, 37 
U.K.4 Range Finder, 37 
Zeiss Range Finder, 176 
Range Finding Sight, Australian, 156 
Ranging, effect on the eye, 150 
Ranging accuracy — Instrumental fac- 
tors; blacklash, 65 
calibration errors, 58 


Parallactic errors, 32, 41 
penta-reflector errors, 41, 50-53, 63 
RCS settings, 44, 57-62 
temperature errors, 35-53 
types of errors, 23 

Ranging accuracy — Psycho-physiolog- 
ical factors; see also Stereoscopic 
acuity 

aniseikonia, 73, 82 
atmospheric effects, 95 
binocular fusion, 69 
binocular vergence, 69 
blurredness, 96 
bracketing error, 99 
chromatic defects of the eye, 

97 

“clipping” errors, 32 
colored filters, 63 
curve B, 95 
cyclophoria, 70 
drug effects, 78 
eye pupil size, 79 
eyepiece focussing errors, 101 
height of image error, 102 
Interaxial distance, 31-32 
intermittent visibility of low contrast 
targets, 98 

interocular settings, 29 
interruption of practice, 75-76 
leveling and alignment, 63 
loss of contrast, 95 
loud sounds, 76 
low illumination, 80 
low oxygen, 80 
magnification, 54 
operator fatigue, 70 
parallelism of rays, 64 
perspective error, 23, 25-34 
posture, 80 
psychological bias, 65 
relation of reticle to target position, 
82 

sex differences, 77 

startle, 79 

target off center, 9 

tracking error, 6, 82, 99, 159, 173-174 

unequal light transmission, 64 

variations in blood sugar, 79 

Ranging and tracking simultaneously, 
158, 173 

Ranging principles, general discussion, 
6 

RCS settings, 44, 57-62, 94 

Reaction time for binocular fusion test, 
129 

Recommendations for range finder op- 
erator training, 140, 144 

Recommendations for improvement of 
range finders; bracketing, 99 
calibration, 59-62 
change-of-power lever, 43 
checking range finder focus in the 
field, 48 

electrically heated covers, 47 
elimination of psychological bias, 65 
height of image adjustment, 102 
helium charging, 40 
interpupillary adjustment, 29 


invar optical bars, 49 
IFD measurements, 114 
leveling and alignment, 63 
magnification, 54 
ortho-pseudo range finder, 177 
penta-reflectors, 51, 52 
perspective error reduction, 25, 33 
range finder for armored vehicles, 
152-155 

ranging with reduced contrast, 96 
RCS settings, 58 
reticle design, 83, 121, 124 
reticle inspection during manufac- 
ture, 162-163 

tracking and ranging simultaneously, 
160 

tracking handwheel, 91 
variable diaphragms, 27 
Recommendations for selection of range 
finder operators, 76, 97, 98, 109, 
124, 132 

Recommendation for selection of sub- 
marine personnel, 130 
Recording theodolites, 133 
Refractive error reduction with helium 
gas, 39 

Reticle design principles, 161-174 
absence of fine elevation adjustment, 
170 

approximating zero error, 163 
comparison of patterns, 161, 164-168 
courses used in experiments, 164 
differences in configuration, 165 
effect of eyeball torsion, 172 
effect of stereoscopic movement, 168 
effect of tracking error, 167-168 
factors affecting precision, 164 
false fusion, 170 
for stadiometric ranging, 173 
fore and aft marks, 164 
full-line reticle, 161 
German design, 167, 168 
height adjustment errors, 171 
imperfection in the reticle field, 162 
loss of contact with reticle, 169 
monocular coincidence, 163 
opaque reticle patterns, 163, 172 
recommendations, 173 
reticle inspection, 163 
service reticles, 161, 164-173 
thickness of reticle lines, 171 
Reticle pattern types; break-in-line 
type, 83 

German R-40, 167, 168 
Gray circle reticle, 169 
Navy Diamond, 161, 164, 168, 170 
Navy Line, 167, 170 
Navy Post, 161 
standard .\rmy, 161, 164 
“three-dot” pattern, 167 
vectographic reticle, 96 
Rhodium coating for range finder win- 
dows, 175 
Riggs reticle, 161 
Rorschack ink blot test, 127 

Sex differences in range finder use, 78, 
117 


INDEX 


199 


Sherman tanks, use of range finder, 152 
Short Base Range Finder, 149 
Shuron interpupillometer (IPD gauge), 
31, 98, 113-114 
Slant range errors, 137 
Slide rule for range finder operators, 145 
Space eikonometer, 73, 123 
Specifications for satisfactory range 
finders, 175 

Split field coincidence type range finder, 
17 

Spotting with the height finder, 103 
Stadiometric ranging, 158, 173 
Startle, effect on ranging efficiency, 79 
Steadiness test, 112 

Stereo camera method of measuring 
IPD, 114 
Stereo strips, 5 

Stereoscopic acuity; see also Ranging 
accuracy — Psycho-physiological 
factors 

effect of altitude, 12 
effect of distance, 11 
effect of magnification, 14. 15, 55 
end-point analysis, 16 
factors affecting, 10-16 
just noticeable difference (j.n.d.), 13 
relation to accommodation and ver- 
gence, 13, 98 

relation to visual acuity. 115, 124 
sex differences, 1 1 7 
telescopic vision, 14 

Stereoscopic acuity tests; Bausch and 
Lomb ortho-rater stereoscopic 
test, 125 
Bott test, 117 

Dearborn-Johnston test, 125 
dynamic tests, 118 
eikonometer test, 107-112, 116, 123, 
125 

Howard-Dolman test. 111, 116 
Keystone tests, 117, 125 
M2 Trainer test, 108-112, 116, 121,125 
Projection eikonometer, 125 
static tests, 118 

vectograph pursuit test, 109, 117, 119, 
125 

Verhoeff size-confusion tests, 117 
with Tufts Trainer, 117 
Wulfeck vectograph test, 117, 125 
Stereoscopic Height Finder school, 97- 
98 

Stereoscopic Observers’ Course, 135 
Stereoscopic range and height finder; 
see also Range finder design and 
performance. Range finder oper- 
ator, Range finder operator selec- 
tion tests. Range finder optical 
systems, new. Range finder types. 
Range accuracy 

compared with coincidence range 
finders, 17 

Stereovertical test, 108, 109, 123 
Stevens Institute, 76 
Stratification effect, 38 
use of helium gas, 40, 41 
Sun, effect on elevation error, 39 
Sunshade for height finder, 42, 47 


“Super Range Finder”, 178 
Superimposed type range finder, 10, 178 
Synoptoscope, 70 

T16E1 Range Finder, 158 
T25 Range Finder, 151 
T26 Range Finder, 151 
Tanks, range finders for, 151-152 
Target aspect, effect on range accuracy, 6 
Target position, relation to reticle, 82 
Telescopic vision, stereo-acuity of, 14 
Telestereoscope, 14 

Template for setting acular separation, 
33 

Tests for selection of range finder oper- 
ators; see Range finder operator 
selection tests 
Theodolites, 133 

Thermal effects in ranging instruments; 
causes, 40 

control by air stirring, 36, 46 
control by electrically heated jackets, 
41,47 

control by helium charging, 38, 43, 44 
control by sunshades, 42, 47 
control by temperature tubes, 36 
effect on accuracy of instrument, 42 
elevation errors, 35 
foggings, 41, 47 

in coincidence range finders, 35 
in stereoscopic range finders, 38, 51 
on mirror surfaces, 50 
on optical bars and penta-reflectors, 
49-53 

penta-reflector errors, 50 
stratification effects, 39-41 
Thermocouple installation in Ml Height 
Finder, 38 

Thickness of reticle, 171 
“Three-dot” reticle pattern, 167 
Torsion of eyeballs, effect on reticle 
design, 172 

Tracking; angular tracking errors, 84 
antiaircraft tracking, 92-93 
continuous tracking, 99 
criteria for selecting operators, 86 
errors, 6, 84, 159, 174 
experimental tracking trainers, 91 
fatigue, 86 

handwheel, 85, 87-90 
individual differences, 85, 87 
inertia, 88-89 
magnification, 91 
operator learning curve, 85 
sex differences, 86 
slewing sights, 92 
target speed, 90 
tracking sights, 92 
with simultaneous ranging, 158 
with untrained operators, 88 
Tracking telescope combined with con- 
trast meter, 96 
Tracking trainer, 84, 91, 140 
Tracking with foot ranging controls, 
160 

Training instruments for range finder 
operators; Brown Stereo-Trainer, 
77, 138 


Howe Laboratory training instru- 
ment, 138 

M2 Trainer, 76, 116, 138, 140 
M4 Trainer, 139 
M6 Trainer, 9, 139, 161 
Mark II Trainer, 29, 30, 70, 76, 77, 
82, 101 

Mark 4 (Eastman) Trainer, 138-140, 
157, 158 

score computing aids, 145 
Tracking Trainer, 84, 91, 140 
Tufts Director Tracking Trainer, 84, 
91, 140 

Tufts Stereoscopic Trainer, 74-78, 82, 
99, 117, 138 

Training manual for range finders, 74 
Training manual for stereoscopic height 
finders, 142-144 

Training program for range finder 
operators, 140 
True target position, 133 
Tufts College; constant error, 99 
diopter settings of eyepieces, 101 
effects of fatigue and motivation on 
ranging performance, 70, 74 
effects of loud sounds on ranging and 
tracking, 76 
interocular settings, 29 
relation of reticle and target position, 
82 

reticle design, 161 
sex differences in ranging, 77 
Tufts Director Tracking Trainer, 91, 92 
Tufts Stereoscopic Trainer, 74-78, 82, 
99, 117, 138 

Two-hand coordination test, 129 
Twvman-Green interferometer, 50 

U. B. 7 Range Finder, elevation errors, 
37 

FI. D. 4 Range Finder, 37 
U. K. 4 Range Finder, 37 
UOE (units of error) recording device, 
7 

Vectograph pursuit apparatus tests, 109, 
117, 119 

Vectographic reticle, 96 
Verhoeff size-confusion tests, 117 
Visibility conditions, effect on ranging, 
6 

Visual acuity, relation to stereoscopic 
acuity, 124 

Visual acuity test, 115 
Visual range estimation tests, 10-14, 
112-115, 153 

F’isual tests for range finder operators, 
115 

Willoughby test, 128, 129 
\Vindow stops for reducing perspective 
errors, 25 

AVulfeck group test, 117 
^Vunderlich - Hovland personnel test, 
112, 128, 129 

Zeiss Interpupillary Gauge, 31, 113 
Zeiss Range Finder, 167 



4 




s' , 
1 







By Mitliarity SnnIux g| 


, SEP 14 1960 

n wBo 2 A ugust I960 
UBRAB Y OTNOBBSS 














